We just got back from the toy store and I have my new CamelBak Rocket.
After solving it’s expandable secrets, it’s its (please don’t shoot me, grammar nazi) decidedly the best combination of water/stuff storage with the most ventilation.

I’m testing it out right now, and despite my lovely wife’s mocking, I’ll probably leave it on while I’m doing some house cleaning in preparation for guests – I don’t want to dehydrate during all that hard work.