We don't have many bath toys, but this activity has shown no signs of getting old, because there are endless creative possibilities! I'm quite lucky in that whenever my friends get kid's craft kits that involve craft foam, they always give me the left overs :) My house has become the repository of unwanted random bits of craft foam and that's really handy for bath time!
I know you can buy chunky foam bath "stickers" that adhere to the tiled walls or side of the tub with just water due to surface tension, but they are pretty limiting in that they are all pre printed themed pieces, so there's no creating for the kids beyond imaginative play with the set they have. What you want is a load of brightly coloured regular craft foam. Not the self adhesive backed stuff, just the cheaper plain old stuff. The Dollar Tree often does decent sized packs with a few colours in and really, the cheap thin stuff is the best because it's easier for the kids to cut and is lighter, so the surface tension keeps it where they put it for longer. If I leave the mural up after their bath in the evening then only about half of it has dried to the point where it falls off into the tub by the morning!
Sometimes I cut out sets for the kids to use like the monster set that you can see below...
Other times I use up random left overs like the insects and flowers for the garden set. The kids like to be able to cut their own pieces too. The only rule is that if you're holding the kid scissors then you're sitting down. Some of the pieces like the monster eyes are drawn on with sharpie pen and they hold up pretty well to getting wet. If you have any of those shaped punches meant for scrap booking then you can get a load of good shapes quickly from those. I only have a punch for making rounded corners, so we just hand cut stuff with the children's safety scissors.
You can go geometric and abstract with shapes (this one is a city on a planet in a binary star system)...
We've also got a set based on the Duplo word blocks from a while back.
For younger kids the alphabet might be popular with lower case on one side and upper case on the other?
You don't even have to be having a bath at all, you can just give them a bucket of water to dunk the shapes in and they can stand in the tub with it cutting shapes themselves.
The kids think it's really funny to be sitting in a tub of craft foam soup, and it's easy to clear up. I just store the sets under the sink in the bathroom in ziplock bags (I don't close them though, because I guess they might get funky if they were still a bit wet and sealed shut?) Because the foam floats, you just need to scoop all the little bits off the surface of the water before taking the plug out so you don't have them escaping down your plug hole and blocking the drain.
There are loads of possibilities with this, even for older kids. They could make board games to play on the sides of the tub, or their own poetry set like the magnetic fridge ones you can buy. My youngest daughter up until she was two years old would have just tried biting chunks out of the pieces though, so I guess there's a lower age limit to this unless you can find really thick craft foam to cut yourself.
Some of the themes we'll be going for in the future are things like numbers so we can practise addition and subtraction, along with circles for counters. I think my girls would really enjoy having all the internal body parts cut out with name labels to construct the human body too. Paul suggested robot parts, which I think they would LOVE! Cut out country shapes to fit together a world map? State shapes for a US map if you're American? Tangrams? sections to build wiggly tracks around the inside of the tub that you can drive your toy cars around on like one of those vertical wheels of death? You're only limited by your own imagination with this!