In addition to all the good stuff already mentioned here, I also carry a small first aid kit and a tiny plastic container with a few each of the following: ibuprofen, immodium tabs, tums, Nuun electrolyte tabs. Oh, and a small tube of neosporin. I like it for lubing my thumbs inside my gloves, and it is good for scrapes and saddle sores, too.

And, after getting covered with bugs last week when I was changing a rear flat, I intend to order a few individually packaged bug repellant towelettes. I also have a couple disposable latex rubber gloves in my repair kit...great for keeping hands clean changing flats if you have to look presentable right after a ride (work commutes, etc).

Also, I don't love my Camelback, so after having a blowout tire 40 miles from home one day in somewhat rural country on a hot day, I had a 3rd bottle mount put under my down tube. On long solo rides if there aren't going to be stores around, I'll put 3 bottles on the bike, and take a commercial (disposable) bottle in my jersey pocket, drinking it first. I like having lots of liquids.