I got this idea from Pinterest the other day... and it has been a HUGE hit in our house! Encourage the kids to have a little fun... in the bath... by turning off the lights.
Throw in a few glow sticks (we have a bunch from parties/halloween), so we just pick a few, activate them and throw them in the water for lots of bathtime fun. When you throw them in the water, they really light up. There are three thin bracelet sticks in the tub in this picture.
Give it a try and let me know what you think!
PS - I did a search online and you can find these pretty cheap on Ebay!