I have two camelbaks, a Rogue and a MULE. The MULE is a little bigger and holds a little more water. I use that one when I want to bring a long sleeve t shirt or a few extra snacks. Sometimes I go with friends who dont have bigger camelbaks and I carry stuff for them - but pretty much all of us have a camelbak.

Besides that here is what I take along:
spare tube and patch kit, pump, tire levers, a compact multi-tool, a few powerbars, cash, keys, cell phone, sunglasses.

The Rogue holds all of that pretty well. I could even the the Tshirt in there if I wanted to. 70 ounces of water too...that keeps me out for a few hours.

Of course I wear a helmet as well.

Have fun-