Sassy bathtub appliques
Our kids love these -- foam letters and numbers that stick (when wet) to the bathtub and tile. They invent endless games, from building and sounding out words to stacking towers and "sandwiches." And for the second time, we have a 4-year-old asking to extend her bath so we can spell more words together on the wall. By far the most-fun, most-used "learning toys" we have.
[I have to throw in my vote for Sassy toys as well. Our most-used bath toy is a silly plastic funnel with a spiral inside that twirls when water goes through. Our son played with it from when he was a year old till he was five. Our daughter now plays with it during every bath. Both my kids also loved their rattles and manipulation toys. -- Ed.]