I have a tough time getting DS (3.2) into the bath. If I can entice him in with a new toy it's a lot easier although obviously I can't go buying new toys all the time! So I've been searching through cupboards and recently we have played with:
his teaset
adult teapot (metal)
plastic bottles/cups/jugs
plastic bag inc punching holes in it
various boring things to see if they float/sink eg: stones, wood.
Is there anything else I probably have lying around that might be fun for him?