Like Striker said, Portland is a haul to the Border.  I'd assume you're going to cross at Calais into St Stephen then head up 1 to St John.  I'd recommend crossing at the 2nd Port in downtown Calais and hit the candy factory in St Stephen.  Be careful driving at night or early in the morning.  Swamp Donkeys on the roadway will ruin you evening and the fog can be crazy thick.  St John is a cool town for a few days.  Italian By Night has good food and Lemongrass is a fun bar with live music. Go to a Sea Dogs game if they have one.

Are you planning on going up the coast and back or flying out of Moncton or some other place?  Beware they will stick it up your tail pipe if returning a rental car in a different country.  Also, flying in/out of Moncton or Freddy will be an all day ordeal.  They'll fly you to either Toronto or Montreal then to someplace in the US then to your final destination.  I normally fly into Bangor and drive 9 if I'm going to the Down East area.  If I'm going to Freddy or Grand Falls I hit 95, cross at Houlton/Woodstock and take the Trans Canadian in the respected direction. But those routes do not offer majestic ocean views just pine trees and rocks.

Not a problem.
Portland is full of Massholes so beware. Its the wrong time of year but if you see fiddleheads on a menu, order them. I consider them the North East version of fried okra. The Maritimes are pretty cool, I think you'll like it. NB is funny as the closer to Quebec you get the more French it gets. If you like oysters, my personal opinion is, PEI are the best, better than Gulf Coast. Those West Coast oysters are like snot on a slate.  Order poutine. I'm probably going to catch hell for this but Quebec has the best Poutine in the world.  Some good beer, Alpine is not.
Since you'll be flying back from the US, look into Bangor. The downtown area has some decent restaurants/bars but you'll need a cab to get back to your hotel. The airport has a Sheraton that isn't too bad and has a restaurant that's okay.  Also, Stephen King lives like a half mile on the other side of 95 from the airport. Pretty cool house.
Make sure to top off your tank before crossing into Canada. Call your bank/CC company before going, check your cell plan before leaving, do currency exchange at a duty free at the border rather than an airport, and your going to lose cell service from time to time, so be aware of that for GPS purposes.
When are you headed up?  I didn't get pulled this year in ME for a Moose permit but my buddy's son and his wife both pulled one.  I'll be headed up sometime in October to help out with that and for business.

I exchange USD to CAD on the US side. The duty free in Houlton only charges $1 for it which is way cheaper than any bank or airport. I think the DF in Calais is the same company.  I try to keep at least a few hundred bucks on me because of the international transaction fees my CC company slaps on each swipe.

We just get Canadian money from ATMs when we're there.

With the amount you travel, have you looked into getting a hotel or airline affiliated CC that doesn't charge international fees? I have a United visa, my wife has a Marriott visa. Those are all we use when we're in Canada, no extra fees.

My debt cards have never worked in Canada.
I’ll have to look into those CC’s. I’ve been on the fence about getting a Delta CC since I fly with them a lot. Thanks for the info.