Whatever your reason for going to Minneapolis, do not miss spending at least a day, perhaps several in St Paul. It is smaller but it is also the seat of Minnesota government and it has a lot more history.
Visit the Sculpture Garden, drive down Summit Avenue, see a St. Paul Saints minor league baseball game in the summer (if you can get tickets!), visit the Mall of America if you must, walk around Uptown and Lake Calhoun.
This museum in downtown St. Paul is huge and has displays of everything from the Human body and genetics to Dinosaurs.
Saint Paul is a nice town to wander around in. If you're in the Twin Cities, might as well see them both.
When in Saint-Paul, one should see Summit Avenue. Summit has many early 20th century houses and is a lovely street to walk in.