The Ring of Fire
This works best if you're sharing a toilet with someone, like in a hotel or hostel etc:
Sneak the toilet paper from the bathroom and unravel a few feet of it. Slice up a few fresh chilies and sprinkle them over the paper. Leave it a few minutes for the juice to soak up and dry, then roll up the paper and put it back in the bathroom. Wait 'til someone goes for a crap!