Ideas for a summer project?

So now it's the summer before college starts. My friend and I are entering college as engineering majors, and we want to work on a project over the summer that is in that general field. Any ideas? Cost needs to be minimal, and we have limited access to equipment. Examples: last summer, I gave my electric guitar a makeover and built a scooter out of bike parts and steel pipe. I also have spare bike parts, especially two wheels, that I'd like to put to use. Any ideas are appreciated!