Have you played on Pinterest yet? It's so fun, but a total time suck. Don't say I didn't warn you!
It is full of fun ideas like this one, which was originally "pinned" from this book.
Just add a drop of food coloring (or two for making purple, orange, and green) to your ice cubes.
Pop a few in the tub with your kids and let them chase the cubes around before they melt.
This made bath time go from not so fun to coooooool! Ada now frequently requests, "Ice, bath?"
They didn't stain her or our tub, and the little bit of red on our bathmat (oops - they're slippery) washed out fine.