Learn to solve a Rubics Cube. Seriously... I spent weeks learning how to solve a cube and got it down to less than 90 seconds, so I used it in a song once on stage. Totally freaked people out cause I was singing at the same time.
Don't make a huge deal of it though, just solve it and toss it aside like it was nothing special.

Another freak out is to use your hand as a puppet. Right in the middle of a song, pull your hand up and do ventriloquism to do a few lines, then push your hand away and go back to singing and act like nothing happened.

... then of course there's always the 'reciting Shakespeare' while you do a tapping lead'