For my 40 mile loops (the meat and potatoes of how I ride), I'll carry 2 bottles of water (plus another two back at the car), a sleeve or two of Shot Bloks, and two Cliff Bars, Lara Bars, or other bar type nutrition. I'll only eat one sleeve of the Shot Bloks and one of the bars, but I carry two just in case. I've come across riders in distress before who needed something to get them through and a powerbar seemed to help.
2 tubes, some means to inflate them, a small folding bike tool, and tire levers. The other thing I'll carry is a copy of my driver's license, a copy of my health insurance card, and $11. Why $11? Because you can use a folded $1 bill inside a tire to deal with a cut tire, and the $10 is in case I need a ride or want to stop off at a gas station for something mid-ride.