I did one of those projects a long time ago, but not as detailed as that.
Ok, as in singing? Or building the model (if you're doing that)? I'm not good with the singing part, but for the model I would suggest maybe a floodgate or dam? Because the contractile vacuole removes extra water from certain cells? Maybe that's not the best, but it's all I've got off the top of my head.
Sorry if I misunderstood.