Let’s get the obvious activities listed:
Wash your naughty bits
Wash someone else’s naughty bits
Have that person wash your naughty bits.
I prefer to bathe alone. Then I can get down to the good stuff!
Hum a good hum.
Close your eyes and float.
Roll over and submerge your face to just under your eyes and pretend you are a hippo lurking.
Submerge your feet under the water and bet on which one will slip up the slippery end of the bath first.
Pretend your fingers are fish and swim them about the bath.
Allow the toes of one foot to just emerge above the surface, and pretend the toes of the other foot are crocodiles hunting the first foot.
Float to the end of the bath until your knees bend, then push off from the end of the bath and make a wave.
Clean up the mess before the kids find out.