Make groans and then ask the person in the stall next to you for constipation medicine.
Say interesting, more floaters than sinkers.
Brush your teeth at a sink then while a person is washing there hands and spit your tooth paste in their sink.
Bring a hyper dog into the bathroom ( if the place allows dogs)
Play a game while using the toilet and keep the volume up as high as it will go.
Clap or cheer loudly when ever some one brakes the silence with a bodily function noise!
Make groans then drop a cantaloupe from 3ft high then sigh relaxingly.
Tp your stall.