S.S.
WI Dells is always nice for a "true" vacation--many of their hotels now have such huge waterparks you'll never have to leave the hotel!
If you're looking for something more affordable and closer to home (like, for 1-2 nights), we LOVE the Holiday Inn in Otsego. The water park is a little smaller, but way more manageable for little ones. There is a separate zero-entry pool for really little kids, and even some bigger rides for big kids and adults only (I love the big "toilet bowl" thingie!) Best of all, room rates are afforable, and kids under 12 stay and eat free. Have fun!