I like this thread.
I second the lessons idea. I've been teaching flute lessons since I was a freshman in high school. The money has paid for a new flute, my first car, my books in college and spending money. As an adult it has financed vacations, Christmas and other whims I ear mark it for. You must have a talent for it or parents wouldn't/shouldn't pay you, but if you have the talent you can charge $15/$20 a half hour.
Tutoring
Gardening
Before and/or after school care