What about pet-sitting for hamsters, rabbits etc., or dog walking? Try ringing round local animal sanctuaries or kennels. Ask around at local restaurants if they need washer-uppers....don't bother asking at big 'chain' type places as you aren't old enough to be officially employed.
Maybe small local shops could do with a helping hand to keep the stockroom tidy or unpack boxes? Ask around but have a friend or parent with you when you do. Offer to wash and polish cars on your estate, take a dustbuster with you and offer to vacuum it as well for extra ��, make it look good by printing your own 'price list'. Big carwashing sponges are only 50p for 2 at the minute in Asda.
Sell unwanted stuff on e-bay or ask around your family, see if they have any stuff they want to get rid of.....see if a car owner can help you have a car boot sale. I made over �80 the other week by selling junk from my loft...old books, clothing, bits and bobs.
Put cards in local shop windows and ask people to pass on by word-of-mouth that you're after extra cash. Whatever you do though, make sure you keep safe and let people know where you are at all times.