I think these would make interpreting sound checks much more interesting.
1) Instead of just saying "hello" or something boring, hum different bits of the Mission Impossible theme into the mic.
2) Impressions.
3) Try out bird calls.
4) Sing a folk song badly.
5) Fake having a deep conversation with someone else while "accidentally" leaving your mic on. Say something like "the doctor said it's my own fault for running so hard into that bush" or "look, I've told Bill Gates that seven million isn't enough" or the ever funny "yep, that's three rolls of tape, a hazmat suit, and 27 kgs of sausages. What do you mean they're frozen?"