Looking for a small hydration pack. Just large enough to carry the bladder, boo boo kit, multi tool and a poncho or rain jacket. Suggestions?

What attachment system are you looking for?

Backpack straps, MOLLE, or Zip On?  If Zip On what plate carrier is it going to go on?



Edit: Oh yeah, and what bladder size?

If looking for backpack straps, I love my Eberlestock H7.
Bought it on sale for ~$70ish and it comes with a 2L Source Tactical bladder.
I use it as a day pack/car bag; Has the bladder (pretty sure it'll take a 3L, but not positive), a small med kit, spare batteries/charge pack/charging cords, a knife and leatherman, rain jacket, little cash, and some snacks.

Looks like most colors are $100, but they've got one camo pattern on sale for $70.

Camelback MULE
I have an ancient woodland one with MOLLE, but the newest ones are fine too.

Just get a $20 2L Ozark one from Walmart.
Company bought us a bunch ~6yrs ago. I still use, even though I have camelback ones.
Perfect size when going on a short hike, yardwork, or to the range.

So, I've been in a quest to get my back panel set-up perfectly for awhile now.

One of the conclusions I've come to is that having things MOLLE'd on the back or zipped on is a PITA if you need to access them.  

The solution I came up with was to purchase a Ferro Concepts FCPC V5 with a zip on hydration pack that carriers my Camelbak 1.5 L MIL Spec Crux (I went with this bladder, because I could find the Avon gas mask adapters for them more easily than the Source Hydration ones and my MSR In-Link filter fits perfectly into it).  The Ferro zip on hydration is so streamlined I honestly can't tell I have it on even when the bladder is full inside.  It's easy to toss on a ruck over it and easy to sit down back against something as comfortable as you can be with any plate carrier doing so.

If I need more stuff than just my 1.5L bladder, if I need more water and sustainment gear then I'm going with a pack with straps.  I manage the straps with my MattBock Strap Claws, which work with my FCPC, but would work with any carrier where the Velcro chest panel was MOLLE or laser cut instead of just solid velcro.  Now the advantage to the MattBock is that it manages the shoulder straps to keep them out of the shoulder pocket so that you can shoulder your rifle obstruction free.  

I don't have one right now (I have a Tactical Tailor pack that will soon be retired), but I want to get a Grey Ghost Gear 1 to 3 day expansion pack and then get some zip on adapter so that it can zip on my Ruck.  My goal is to have something that attaches nicely to my Ruck and can be removed from the Ruck quickly and then tossed on behind my carrier and accessed by me when I need it.  The reason I also like the Grey Ghost Gear is that it's expandable and compresses down.

Now, my main complaint about the Ferro zip on hydration is that although they give you an interior elastic pocket for the bladder they did NOT include a hanger on the inside for the bladder, which is pretty disappointing. I do prefer both a hanger and an elastic pocket for a bladder especially since the tiny bit of paracord for a hanger is not that difficult to add.  The Ferro zip on pouches though only fit the Ferro plate carriers, which is another complaint that I have about them...who makes proprietary stuff these days?

...How has nobody mentioned a hsp flatpack?

@KaerMorhenResident

My backpanel is setup to extend my mission time by 12 hours. Nothing in my backpanel us designed to be accessed while needing the plate carrier on me.

Contents
Bladder
Ration
Nuun water tablets
Extra batteries
Spare radio battery
Chemlights
Waterproof match
Smallish amount of 550 cord
Non reflective emergency blanket
Moleskin
Collapsible bottle and water filter system
Baby wipes

As I mentioned-none of this is meant to be used while wearing the carrier (minus bladder). If something goes wrong I have an additional 12 hours to figure out, and execute my next move.

I get it, but what if you want to toss on a full size ruck? How it works with a full size pack is as important to me as how it works with the carrier.

Have tested and works fine.

Source Commander.. comes in 2 sizes both with 3L bladder

Cdnn has multiple versions of the eagle industries source packs for a good price.

https://www.cdnnsports.com/catalogsearch/result/?q=Eagle+source