I've been using a Camelbak Blowfish. It has air channels that keep the back a bit cooler, it holds a full-size Camelbak bladder and directs the tube over your shoulder, and it can be zipped down to a very flat shape or a larger shape (like expandable luggage), hence the name.

Zipped down, I can fit my first aid box, bath kit, and a change of clothes. In full-size mode, a bit more (but not twice as much). Comes in black with silver reflective stripe down the back.