An ever so simple tub toy with endless open-ended play possibilities : Ping Pong Balls. You can shoot 'em in a basket, bounce them off the walls, watch them float, catch them with a "net", push them under and watch them "pop up"...let your imagination float freely!
I bought an inexpensive plastic kitchen strainer with a handle that Buga uses as a "net" to "catch the Ping Pong Fish!" She loves doing this while counting them. She also took her bath crayons and drew on some of them! Brilliant!