Science Museum has this cool Body Worlds exhibit. You don't have to pay extra, but you have to make a reservation as they limit the number of people in the exhibit at a time.
http://www.smm.org
Saint Paul Saints are the minor league baseball team in town. Tickets may be available, or not. They are pretty popular. Lots of antics and entertainment at the Midway Stadium.
http://www.saintsbaseball.com
Near the Mall of America, there's this new water park...I think it's called something real creative like The Water Park of America. I know there is a giant water slide that comes outside the building and goes back in.
http://www.waterparkofamerica.com
I'm not into Valleyfair but here you go
http://www.valleyfair.com
Stillwater is a great place to check out if you like antique shops and stuff. Depending on the time you are coming, there's a really fun apple orchard called Aamodt's Apple Farm there. Great family business, good apples and other stuff. They also have the Saint Croix Vineyards on property...Minnesota wines. They have a couple good vintages there.
Speaking of that, there is a new Minnesota Wine Trail that was just being publicized...kind of a way to have people visit all the wineries in MN, or at least the southeast section of the state. You probably won't have time for the whole shebang, but see if you can find info on that...you might have time for something in Stillwater, or something close to Northfield.
Again, dependent on when you are up here, we have the Taste of Minnesota, which usually ends around the 4th of July. It's a giant food fest with bands and vendors and stuff usually at Harriet Island in St. Paul.
Check out the MSP Convention and Visitor's Bureau. You can probably get an idea on what's happening on the dates you are here from them.
