Here are some ideas we have enjoyed:
Food coloring: create a one color bath or pour different colored water in squeeze bottles and let them add it
All cups; different shapes and sizes
Homemade bath fizzies (aka alka seltzer tablets) :)
Glow in the dark bracelets (must admit my 3 year old didn't like them in the tub)
Snap lock beads (and they make great microphones too)
Plastic farm animal figurines
Winnie the Pooh set of figurines
Plastic little pony figurines (and they get washed as a bonus)
Bath mirror and silly shampooed hairstyles always gets a laugh
Polka dot bath with foam craft circles (warning-don't leave them on your bath wall overnight-the colors may bleed)
The theme I have pictured below are the foam craft circles you can buy at Dollar Tree. I put about 12 drops of blue food coloring and we pretended the green foam craft circles were lily pads and balanced bath cups on them.
Next up for us will be homemade bath crayons created by grating bar soap, adding food color and molding them into the shape of big crayons. What is your kid's favorite bath time fun?