I don't know where you are or what the laws are, but check out if these ideas are legal.
Washing cars.
Doing housework.
Cooking and selling cakes, sweets and other goodies.
Walking Dogs.
Clearing out pet's cages.
Running Errands.
Keeping an eye on smaller kids. (keeping them amused while an adult is within calling distance)
Writing a story or article for a paper or fanzine or magazine.
Growing and selling plants.
Selling off old things you don't want anymore.(Be careful of this though. You might not think you want your Sindy doll anymore, but you might again later.)
Helping younger or struggling kids with their schoolwork.
Giving Mum or Dad a massage.
Sewing or making nice things to be sold.
Helping with decorating.
Helping out at a local shop.
Paper round.
Doing some art work, like a mural somewhere.
Cleaning off graffiti.
Put on a play, concert, or make a film and charge for folk to see.
See if any local businesses need some help.
Help an older person understand the Internet and computers.
The most important thing is to make sure that whatever you do, an adult knows and keeps an eye on things.
Good luck!