Something to eat. You can always buy something at a convenience store, but it is a good idea to carry something with you just in case there isn't one there when you need it. I usually take peanut butter crackers, fig newtons, and a GU or two, depending on the length of the ride. I aim for 100-200 calories per hour after the first hour of riding.

Also have some id on you with emergency contact information.

Have fun!