I always carry my camelback. Even when road riding. Roadies seem to prefer bottles, but I don't handle the bottles well, so I use my camelback. The tube is always there. I think I drink more using it.

Contents:
Pump
Tire irons
Patch kit
Park mulit-tool thing with a chain breaker
Spare chain pin
Zip ties
Small first aid kit
Cell phone
Gu or bars
ID and
Insurance card.

And I keep a spare tube wrapped in an shop rag. This helps keep the tube from getting punctured while in the pack, and also, I always have a rag handy if I need it.