Get powdered tempura poster paint, the cheap kind—preferably in red or brown. Unscrew a shower head, pack the paint into the pipe, screw the shower head back on. When the person turns on the water, it looks pretty scary, especially if it’s red :D
Tightly saran wrap the toilet between the rim and the seat.
If you have an old mask, put it between the rim and seat with the hair taped to the bottom of the seat lid, then close the lid. When they open the lid to pee, they get a scare.
When I was in high school, my friends and I would go through the mall parking lots and put condoms on hood ornaments. The 80’s Oldsmobile hood ornaments were the best—and watching the senior citizens drive, stop, then get out of their cars to check was hysterical to us teenagers.