I carr medications, toiletries, a book and a magazine, a small notebook and several pens (I'm always losing them), camera and film, gum (for when my ears "pop" on the plane), my itinerary and travel info.
I usually bring bottled water, too. I am always thirsty, and the plane especially tends to dehydrate me. I always plan to pack a snack, but I always forget. Maybe I should go stash some granola bars in my bag now, as I am almost packed!
Edited because I just jumped up and stashed 2 granola bars, 2 cereal bars, 2 cheese and crackers packets, and a bag of dried mango slices in my carry-on. We won't eat all of this on the plane, but it will be nice to have a few snacks in the room to tide us over when we take a break.