Get a job. It is awful late to be looking for summer work though.

Mow lawns, wash cars, babysit, etc.

1. Sell things you no longer need. Dig out that old guitar you never play, those CDs you don't listen to anymore, or your antique toy collection. Go to the pawnshop, put an ad on local internet classifieds, or hold a yard sale in front of your house or on a busy street corner (just display the item with a big sign announcing the ridiculously low price). Price items at half of what somebody would be able to buy them for anywhere else, and you may be able to sell them within an hour or two.
2. Sell crafts. If you're a dab hand at making something quickly that people want, do so and sell it either online (sites like Etsy are great for handmade goodies) or at local craft fairs or other community-based sales.
3. Sell stock photos. Stock photos are relatively easy to take and, once you’ve posted them to your stock-photo account, can make money fairly passively.
4. Sell scavenged valuables. If you have a metal detector, go to public places (especially parks, beaches, and other areas where people are likely to drop something unnoticed) and look for jewelry and other goods.
5. Sell things walking about the roadside. Entrepreneurial sorts sell things like cold drinks on hot days or hot nuts in winter. But it can be dangerous work and it may be illegal, so check the laws and watch the traffic with great care. A safer option is a roadside stall, where people have to park the car and come and see your wares.