DS got the children's zoo, the meerkat set and the dolphin basin for his third birthday. He loves them all and they're not quite as fiddly as some of the other ones I've seen (except the meerkat food).
We didn't pay that price for the dolphin basin though - it was half price in toysrus.
I think that sets that they can relate to work well (i.e. DS went to a few zoos, petting farms, dolphin/sea lion displays over the summer).