Something else I thought of (tonight in the shower with DS!) he has a plastic ice cream scooper and matching plastic cone. They are actually sand toys but he has one set int he bath/shower and the other for the sand. He likes making soap or rinsing shampoo into the cone to see how high he can build the cone. When he was younger we had three little plastic balls (outside toys, not specifically bath toys) but they fit well into the top of the cone, and he played ice cream shop for HOURS in the bath.
When we were kids my mom always used to give us the hand egg beater to play with. I remember she bought my sister one on her DD's 1st birthday and all of my sister's children loved it too. I looked for one when DS was born but don't know if they just don't make them any more or if they are just a North American "thing" that I can't in Europe.
https://www.google.de/search?q=hand+egg+beater&rlz=1C1ASUT_en-GBDE413DE413&es_sm=93&source=lnms&tbm=isch&sa=X&ei=BwdxU5ebIeuu7Aa6hoGgAw&ved=0CAgQ_AUoAQ&biw=1366&bih=653&dpr=1Ours was like the picture in the top row, 2nd from the left. Totally 1970s style!!
But in the bathtub with bubbles, OMG a child's dream, I think I played with it until I was about 9
