I always carry a backpack around with me, and it contains:
- wallet
- pen (usually more than one)
- folder
- model releases (5)
- grey card
- cell phone
- some food/water
Other than that, it changes depending on what I'm doing. Sometimes I'll throw a tripod into the mix, or filters, or whatever. I try to travel with as little as I can. I've historically brought my 30D with the kit lens on it, but lately I've been more inclined to bring the 70-300. It's bigger and heavier, but seems to be the right lens for what I need more often.