You can also sell books that you are done with or have already read. You can ask your parents for help with pricing the items that are for sale. Be sure to clean up the items before you try to sell them, and never sell any of your things without your parent's permission. If you are a creative person, you can make things to sell. You can make birdhouses, windchimes, potholders, picture frames, etc.
Popsicle sticks are good for making picture frames, and they're cheap, too.
Also, try coming up with your own ideas for things to make. This stuff is great for selling and fun to make. Again, ask your parents for help pricing the things you make. These things take time and work, and I'm sure that if something is waiting to get done, someone would be happy to let you help. There's also recycling.