Don't forget paper towels and spray cleaner.

Another thing I'm never without is a good rag, usually an old t-shirt or something. Very often it's stuck through one of my belt loops always ready for service. That's one of the thing's very often overlooked by new flyers, they don't realize how messy the hobby can get, especially breaking in a new engine...and it's not unusual to see them hunting for something to wipe their hands with.

As for spray cleaners, generic brands from the Dollar General store are a LOT cheaper and work just as good.