Now how did that ...

Thing to do in the bathroom stall...
Say, "Now how did that get there?"

http://www.101funjokes.com/bathroom_stall_jokes.htm

Boy, that sure looks ...

Thing to do in the bathroom stall...
Say, "Boy, that sure looks like a maggot"

http://www.jokes-db.com/jokes/bathroom-stall-jokes

C`mon Mr. Happy...

Thing to do in the bathroom stall...
Say. "C`mon Mr. Happy! Don`t fall asleep on me!!"

http://www.jokes-db.com/jokes/bathroom-stall-jokes

Using a small squeeze tube...

Thing to do in the bathroom stall...
Using a small squeeze tube, spread peanut butter on a wad of toilet paper and drop it under the stall wall of your neighbor. Then say, "Whoops, could you kick that back over here, please?"

http://www.jokes-db.com/jokes/bathroom-stall-jokes

Interesting....

Thing to do in the bathroom stall...
Say, "Interesting....more sinkers than floaters.