K.R.
We've been to the Kalahari and Grand Marquis in the Dells. The Kalahari was really fun for my 4 year old - she loved it! She was 3 when we went to the Grand Marquis. It is small and pretty inexpensive. I liked that it was small when she was little so she could enjoy the slides but not be overwhelmed.
I've also heard great things about Great Wolf.
I've also been to one in Eau Claire - Chaos Waterpark - attached to the Metropolis Hotel. It was mid-sized and pretty fun. There was a lazy river and a small area for litle kids, plus an arcade area.