Take a large sheet of plastic wrap and cover the bowl of the toilet. Naturally you'll want to do this below the seat and the lid.

I've found that it's best if you lay the layers of plastic wrap from side to side rather than front to back, else your victim might catch on before the joke is done. Another good tip here is to leave some slack across the bowl, unless of course you don't mind having the victim's pee or barf or whatever run off the plastic wrap and onto the floor - probably because it isn't your house, but just in case it is, be careful and good luck