Okay here are some ideas, assuming you're in middle or high school :
1. How do aeroplanes stay up?
2. Do-it-yourself DNA
3. A basic block-and-tackle diagram of a pulley
4. Verifying Benjamin Franklin's hypothesis
5. Reflecting periscope
6. Showing refraction in water
These are just some of the ideas. You can think of many others, just developing on these. Good luck :-)