Ice cube tray[s]
Small Bits and bobs from the bottom of the toy box
Coins, buttons, marbles
Freeze each item in its own compartment in the ice cube tray. Fill with water. Add a small amount of colouring. Stir. Freeze.
These work well in the tub or when 'puddling' in the sink. Each cube slowly reveals its own surprise. It's also an opportunity to talk about water and it's different forms and characteristics.
Note: If you get too much colouring in the cubes they can be a bit messy. No harm done but if some of the colour gets on skin directly [before being diffused through the tub water] it can leave a temporary mark. Something to think about if you are having your wee one in wedding photos the next day or something.