Im a large fan ofsimple instruments. The ones that take little or no effort to make sound (such as Kazoos and the like) are the ones that I have found to be the most fun.
Example, my friend and I were at the Flash Forward conference last year. They gave out branded Kazoos for fun. For a half hour, my friend and I went around downtown Seattle proudly (and loudly) playing the theme from Rocky.
The hand drum, a simple drum that mostly resembles an upside down mixing bowl, is one such instrument. It takes skill to become good at it, but for anyone, they can start banging on it to make some music. Example video below:
Since I am also involved in the SCA (a medieval reenactment group) this would be a great instrument to tote around an event and just have fun with. Now only if the local music shop carried them, that would be killer.
-Cheers