The "guess what it is" puzzles are very common around electronics blogs, (see bunniestudios, hacked gadgets...) I think you can make something more original. For example, hacked gadgets has a "guess what it isn't" contest, where you have to let your fantasy work. But I have an Idea for you, since this is an EE blog: show us a product, fully assembled, tell us what it does, and ask us how we would have build it (components, techniques, etc). Then compare with the original and see who got a better(or more similar) solution!
What do you think?