Now that we've got venues, we need musicians!
Each instrument would work somewhat like a cross between a music box and the firefly whistle. This might get a little complicated, but it would be REALLY COOL.
So, you could play each one either just for yourself, or "for the crowd."
Each instrument would have about three tunes or rhythms that would be chosen randomly each time you play one. They might not always sound fantastic together, but most combinations would be entertaining at least.
They give bonuses like music blocks, but when played together "for the crowd", they'd give extra bonuses, including some kickback to the players!
The stage would be a piece of furniture you could put in your towers. Playing "for the crowd" might be limited to only when you are on the stage, or it might not, depending on whether you need to control how often that is done for game mechanics or server issues.
This idea, of course, would go very well together with the tip jar!