R.
I was going to suggest Wisconsin Dells as well, but I think you need a couple of days for that. A friend of mine took his 15 year old son to the house on the rock (I think it's about 3 hours from St Paul) and said it was phenominal! The house is cool and all, but what is the best part is the museum they have there. He said he could not believe the stuff that was in there. His son really enjoyed it too.