As a strange variant, I saw the keyboard/bandleader bang on the bassists strings with fat, bright yellow plastic bats.
But the stage antics I have been responsible for were, beyond general goofing, playing amusing interludes or doing voices and odd annoucements to entertain the audience in between songs (been in a lot of instrument switching bands)
- I once consumed some large peices of a huge raw portabella mushroom for visual effect
- there were several kazoo-inspired episodes, the first of which was the funniest because the audience was not at all into it. I had worked it out with one of the merchant/craftsmen travelling with us to toss out large numbers of small plastic kazoos to the audience at the start of a bouncy early jazz cover we were doing towards the end of the set. It was more like the audience was being pelted with unwanted kazoos - the whole tossing part of it was ill concieved

- we had kazoos ourself onstage and were getting progressively goofy in the extreme, but no amount of effort could get these people at all amused, and it was just a disaster.
I still haven't pulled off the kazoo thing as well as envisioned, but, you know, you got to hone these things.