1. Violently beat against the stall walls screaming, "BEES...OH GOD BEES...."

2. Say, "uh oh, I knew I shouldn't have put my lips on that."

3. Cheer and clap loudly every time somebody breaks the silence with
a bodily function noise.

4. Say, "Damn, this water's cold." Pause a moment and say, "And Deep"

5. Drop a marble and say, "Oh no! My glass eye!"

6. Say, "Hmmm, I've never seen that color before."

7. Grunt and strain real loud for 30 seconds and then drop a
cantelope into the toilet bowl from a height of six feet. Sigh
relaxingly.

8. Say, "Now how did that get in there."

9. Say, "Humus. Reminds me of humus."

10. Fill up a large flask with Mountain Dew. Squirt it erratically
under the stall walls of your neighbor's while yelling, "Whoa! Easy
boy!"

11. Say, "Interesting............. more floaters than sinkers.'"

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

13. Say, "C'mon Mr. Happy! Don't fall asleep on me now."

14. Fill a balloon with cream corn. Rush into the stall with your
hand over your mouth and let out a lengthy vomit impression while you
squeeze the balloon and splatter cream corn all about. Apologize
profusely and blame it on the fettucine alfredo you had for breakfast.

15. Say, "Boy, that sure looks like a maggott."

16. Say, "Damn, I knew that drain hole was a little too small. Now
what am I gonna do?"

17. Play a well known drum cadence over and over again on your butt
cheeks.

18. Before you un-roll toilet paper, conspicuously lay down your
"Cross-Dressers Anonymous" newsletter on the floor visible to the
adjacent stall.

19. Lower a small mirror underneath the stall wall, adjust it so you
can see your neighbor and say, "Peek-a-boo!"

20. Drop a D-cup bra on the floor under the stall wall and sing "Born
Free."

Bonus: Fold a ketchup packet in half and leave it on the nubs under the toilet seat, then sit in the other stall and wait. (thanks to DevilDog82)