There is an old prank where you stretch out some clingfilm over the toilet pan (under the seat). The effects are obvious, and somewhat disgusting.

I've only ever heard of two people doing this, and only ever on guys. I'm sure it works on females, but I'm not going to test it!

Buy some rip-proof toilet paper. Replace the dispensers in your bathroom.

Wait

Listen for the reaction!

You can make rip-proof toilet paper by taking a roll of normal toilet paper, unrollig part of it, and running a line of selotape along it.

Take a new toilet roll. Roll it out a bit. Take some talcum powder and sprinkle on the unrolled toilet roll. Roll the paper back up with the talcum powder inside.

Wait for a victim to go to the toilet and unleash the powder.