What should I make for less than $500?

I have been really bored lately and I want to build/make something. The problem is I don't know what I should make. I don't want it to be useless (ie. wrist mounted flamethrower) or involve chemicals that might be dangerous and/or difficult to obtain (ie. Potium Perchlorate for those cool ninja smoke things) I also don't want it to cost a whole lot of money (ie. powered exoskeleton). All of the things in brackets I actually began to make (even made it to the schematics) but then realized that they were: useless, hazardous etc. I was thinking something along the lines of a robot, but I can't think of a function for it. If the robot gets too big though I might as well work on powered exoskeletons (my design was fairly simple and may cost less than 500$, but emphisis on 'may') I am gifted in the areas of engineering and would prefer to do something along the lines of that. Thanks in advance!