Practical Jokes.

 1... An old classic.
Place cling film (plastic wrap) across the toilet bowl and lower the seat.
2...
Add a thin layer of Vaseline to the toilet seat. Your victim won't know what it is.
 3...
Wait until you're sure the toilet will remain unused for at least 24 hours.
Get 2 - 3 packets of powdered jello and mix it into the toilet water until dissolved.
Let it set. (The more jello you use the harder it will set)