This is a nice simple prank that is quite funny.
What you will need:
1 roll of cling film or plastic wrap
Pencil or pen
Cut two roughly square pieces of cling film.
Wrap the two pieces of cling film around the toilet bowl (lift the toilet seat out of the way first ). Try to make it as tightly stuck as possible. Put one piece near the front and the other near the back so that it covers everywhere.
Write "April Fools!!!", or whatever you want and then stick it on the inside of the toilet lid with sellotape.
You're finished! Who ever goes to the toilet next will get a big shock.