When I'm going on a trip where I will need more than my phone, wallet and keys, I carry a small Timbuk2 messenger bag. I have the Pee Wee, but it looks like one of their new models is pretty cool too, the Metro. My bag is 6 years old and looks like it is brand new. It's very well built, I'd dare say bombproof. I found out about their products from a friend of mine who was a bike messenger in DC for a summer.