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!
Grits
2010 Trek 5.2 Madone WSD, SI Diva Gel Flow
2002 Terry Classic, Terry Liberator