Here’s another one for my own reference, inspired by glancing at the article on ARGs in the October Smithsonian.
Cryptography is a great interactive topic for a science museum exhibit. I mean, puzzles and codes are age-scalable, if anything is. How about an interactive game that codes into a fake Numbers Station? Either shortwave or low-powered neighborhood radio (perhaps even just the museum itself).
If you solve the code, you get a cheap prize. You could even run different codes set a different skill levels.