LED faucet light "changes water color" when too hot
I'm not sure how much of a hack this is as it requires buying a gadget - but for us gadget lovers, that's never a problem. :)
ThinkGeek sells an LED Faucet Light that turns the water blue until it hits 89 degrees at which point it turns red.
Since baby bath water is supposed to be 90 degrees or less, you could use it to make sure you weren't pouring water that was too hot. Also, it could come in handy when teaching an older child how to pour their own bath water. Our oldest used to scream "It's too hot!" or "It's too cold!" as he seesawed between extreme hot and extreme cold