Last year I did Oahu and Kauai. This year it's going to be Oahu, Kauai, Maui, and Big Island. All 4 main islands.

What should I do on Maui and Big Island?

In Oahu I'm planning on sky diving this time and cage diving with sharks. Any other ideas?

There is a LOT to do on Oahu so I'm sure I'm forgetting things.

I've already done lots of snorkeling and lots of the hikes on Oahu, so that aspect is covered.

ETA: I'm also thinking about combining this with a Bali trip. I know next to nothing about Bali though, so that one would be a pretty big leap.

Get a breakfast bomber at kono's on north shore. Those things are amazing.

Feral cattle hunting.

Did you build a clock to get Obama to invite you?

The big island is freaking awesome.  Visit green sand beach, black sand beach, volcano national park, Kohala for zip lining and kayaking, Hilo farmers market, Kona for wine, beer, coffee.  I've been five times for a week or so each and never got bored.  Diving with the mantas is the best dive I have ever done in my life.

It's Kona  Btw, while you are on that side of the BI, hit Kona Mountain Coffee.  Their big place was just up the road from the airport when I lived on the island.  Best coffee ever.  You can tour the Kona Joe farm as well.

On the Hilo side where I lived, you have the Punalu'u black sand beach, Kens House of Pancakes on 5 and Kamehameha in Hilo (great food), Volcano national park, the Volcano winery is pretty good, hit up the Hilo farmer's market on a Saturday if you can.  Cheap grub and Mr. D can BBQ like nothing else. Cafe 100 and Low International are both great.  100 does 100 types of Moco and Low's is just inexpensive good food with fresh made Taro bread if you order it.

You can take the saddle road to the Observatory, though I never made it up there unfortunately.  

Best bet is to take a day in Kona, next morning drive north from Kona to Waimea (beautiful drive) and south to Hilo.  Overnight in Hilo and spend a day there, then take the south road looping the island back to Kona.

Last I heard the Puna green sand beach got hit by lava a year or so ago.  Could have been wrong.  Hell of a nice drive to get there though, and going through that hippy commune was always a laugh.

I think it is Saturdays, but the Orchidland farmer's market is pretty awesome as well.  Just take the T intersection at Keaau and 5 and it'll be on your left in 10-15.  Was a lot faster since I lived in HAA though.

Green sand beach is way down south, a very long ways from any active lava flows.  Pretty fun place to beat up a rental jeep, or a wind blasted hike in though.

https://en.m.wikipedia.org/wiki/Papakolea_Beach

If you are spending that much time in Hawaii, you need to hunt Black Hawaiian Sheep.

http://www.hawaiisafaris.com/Hawaii-Game-Hunting.php

The road to Hana.
Great drive and day trip.

Go to the Dole Plantation and get some pineapple icecream and go in the maze.
Get pineapples shipped to the airport on the day of your departure with their service and you pick them up with your luggage when you get off the plane.

have fun

Big Island:

Snorkel Captain Cook's Monument
Tour Kona coffee fields
Volcanoes NP (highly recommend helo tour)
Drive up to Mauna Kea summit for sunset
Drive to green sand and black sand beach
Go to Hilo and flea market
Zipline over Akaka falls

Huh, good to know.  The ex in laws live out there still and mentioned one of the green beaches got hit by lava.  That was the only one I knew of.  *shrug*

Some days I miss living there.

Lava Flow on a boat on the Big Island... worth every penny
 

Tunnels Beach on Kauai

eat at Tiki Taco in Kauai. its heavenly.

Corrected for Kauai.

I have no input, op. I've only ever been to the islands for work.

I would suggest buying the 4 books in this series that correspond to the islands you're visiting.

https://www.amazon.com/Hawaii-Big-Island-Revealed-Guidebook/dp/0983888744/ref=sr_1_3?ie=UTF8&qid=1476585245&sr=8-3

The Big Island is my favorite. Here are some suggestions:

- 49 Black Sand Beach. This is in a gated community but if you tell the guard you want to go to the beach he'll let you in. It has a reef for snorkeling / spearfishing and 15-20 foot cliffs you can jump off into the ocean.

- South Point and Green Sand Beach. It takes a while to get to this area but it's really unique. It is the southern-most part of the US. If you stand on the cliffs facing south, there's nothing but ocean between you and Antarctica. It is a real adventure to get to Green Sand Beach, you have to hire some locals to take you out there in a 4x4.

- Beach at the Mauna Kea. This is one of the best properties on earth, with a perfect crescent beach and calm waters for snorkeling / spearfishing. At the gate, tell the guard you're going to the beach.

- Lava tour. There is no way to predict whether the lava will be flowing, but if it is, it is imperative that you go see it. It is a once-in-a-lifetime experience and you can actually observe the creation of new earth. You will have to pay a guide to either hike with you out to a lava field or take you out on a boat where the lava enters the ocean. Whatever the cost, you should do this if tours are being offered.

- Manta ray dive / snorkel. There are many dive boats that take tourists out at sunset to scuba or snorkel with manta rays once it gets dark. A ticket costs around $150 and the boats are crammed with kooks who can barely swim. If you're snorkeling, you have to wear a life jacket and hold onto a raft with lights on it, along with a dozen other people. Don't do this. The far better course of action is to rent a dive light from a dive shop ($10/day) and head up to the Mauna Kea 30 minutes before sunset. Get geared up on the beach, and as soon as it's dark, swim out 100 yards to where the mantas congregate. You can't miss it because there is a bright light pointed at the exact spot so hotel guests can watch the mantas from a viewing platform. This way you get one-on-one time with the mantas for as longs you want, it is far superior to going out on a dive boat. To do this, you have to have your own mask, snorkel, and fins, which I assume you will have if you're going to be there for two months. You should just keep them in the back of your car at all times.

During the winter, the north shore of Oahu has mammoth waves and the best surfers from around the world head there to compete. You should go check it out, you may see death-defying type stuff.

stayed a month on oahu as a kid. i really loved it. i'd love to move there when i retire but they're too anti 2A...

^ this.  Also Volcom Pipeline surf competition is usually right after Christmas.

I've spent some cold and miserable nights on the big Island

http://www.mauiexposition.com/MAUISWAPMEET.html

Maui? Surf JAWS

Ohau has a couple on the North Shore that offers powered hang gliding.  Of everything we did on that trip and we did a lot, it was by far the best experience.  The company name is Paradise Air and their website is http://www.paradiseairhawaii.com/

+1 on the hunting.

I made two spanish goat hunts in two different areas when I was there last Thanksgiving. Awesome hunts with goats down quick, and back snorkeling with the kids before lunch both times.

My in-laws live close to volcano national park.

I'd like to euro mount some goats and rams from HI... One of these days I'll be back to do it.

You could do Asia for less than that and have a real adventure.

Maui:

Take an entire day to drive the road to Hana, and stop often to hike the trails on East side of the island.

Take a day to hike Haleakala, watch the sunrise from up there, and spend the rest of the day in the crater.

Maui Brewing Company for food and beer.

Snorkel Airport Beach and molokini crater

Eat at the Fish Market in Paia

See if you can find Alana Blanchard and get her to give you a surfing lesson. Google pics.

What was the approximate cost of the hunts?

Full day was $800 with a guide that only does full days, and half day with another guide was $400.

That's the other question.

I'm thinking about combining this with a Bali trip (Indonesia).

I know next to nothing about Bali though, so that one would be a pretty big leap.

Side Street Inn and Home Bar & Grill. Both near Ala Moana and both damn good food. Family style. Also, Cafe O'Lei in Kihei. Another spot for amazing food.

Fuck Bali, stay away from Muslim countries.  If you wanted another good vacation spot I would recommend Fiji, Palau, or Palawan.

That was my first thought too, but everything about Bali so far just seems more interesting than those other places.

If you like the thought of dying from bathtub liquor, it's perfect.

I think Maui is my favorite, but I haven't been to Kauai. Drive the whole island all the way around on both sides, you find stops along the way and make your own adventure. Spend at least a day or two near Hana, beautiful land and beaches. A few hidden water falls, it's all lovely.

Cant talk about Bali but I would definitely try Asia out.  Its a lot of the things I like about Hawaii.  In some cases even more so, and in other cases much less.  Like price. The flight sucks ass, and theres some annoyances, but I dont think Id go back to Hawaii other than if there were time constraints or something like that.

I spent a week hiking and seeing as much of Kilauea as I could on the Big Island. I'm from here and the sheer size of the park and the amount of destruction and new life is humbling. Feels like being on the moon sometimes.

Get a good mask and snorkel set.

Swim with the rays on the Kona side of the big Island. http://www.mantarayshawaii.com/mantaexperience.html

Ocean kayaking/whale watching west side of Maui.

We just got back from 2 weeks in the Big Island and Maui. Both were awesome and we'd love to go back at any time.

The best thing about Maui is the Snorkeling, check out all the spots. We really enjoyed our 5 hour tour from Blue Water Rafting ... got to snorkel at Molokini and Turtle Town among other places.  The whole west side of the island is awesome, the trip around West Maui was really cool, and I enjoyed a lot of the side-trips like the "blowhole" and the Olivine Pools.  We even did a geocache to a hidden waterfall that involved some crazy stuff to get to the end-  ended up cliff jumping ~20' into a freshwater pool - that was exciting. Also spent several nights just watching the sun set from the beach on the west side- totally worth it.  Maui Brewing Company was a nice place to chill.

The Big Island was really neat, although we ended up doing a lot of driving. I enjoyed the Southernmost point / green sand beach. Unique area with strong waves. Some incredibly hot European gals there. Snorkeled with Manta Rays (that was OK... manta rays are neat) and saw hot lava entering the ocean.

Only weird thing was I was battling motion sickness for the first week we were there, really put a damper on things.

Whatever you do, don't pick up this guy:

Another thumbs-up for the big island.  Went earlier this year, spent a great day just bombing around in the rental car... I think every kind of terrain I've ever seen exists on that island.

As for Oahu... go see the Museum stuff at Pearl Harbor and the Pacific Air museum... then get the heck off Oahu.  

Go fish out of Lahaina when you are in Maui.

Big Island.

Check out caldera at Mauna Loa.  Saddle Road from Mauna Kea is easy route.  I hiked it, 21 mi, one way, 2 days up, one day down.  Two cabins for shelter, one half-way, one at top.  6700' altitude gain.
Think there's a winery on big island, Volcano Winery.
Dr.Seuss art at gallery on Big Island.  
Parker Ranch headquarters at Kamuela/Waimea in the hills.
Reef walking in Hilo
Get the books, Hawaii Revealed, Maui Revealed etc.

Hmmm...humble brag post much? ONLY two months "this" time? Sounds like you've been there before and shouldn't need help from GD to decide what to do.  

Most are lucky to spend a week there in their lifetimes.

I'm on the big island now.
Volcanoes
Waipio valley
Two step for snorkeling
Broke da mouth grindz (Hawaiian food)