The Milwaukee Public museum is hailed as one of the ten best in the entire country and it's easy to see why. I've been goign there since I can remember and it has never failed to surprise me. Everytime I'm there I see something I hadn't seen before. It's huge, it has a great interactive discovery museum for kids, it has a complete replica of a downtown Milwaukee street from the 1800's, as well as my favorite, their awesome dinosaur exhibit.
The museum also, a few times a year, has different special limited run special exhibits. These are typically well put together presentations that can teach your child something you didn't know... and probably even you.