We love the Science Museum of Minnesota!
They have wonderful permanant attractions like the Art Park and Mini Golf, Dinosaurs, Mummy, Electricity etc... and at least 1 rotating main attraction. Currently it is Body Worlds. [Incidentally, this one is pretty gross, I didn't care for it]. They also have the Omni Theater which is amazing for watching educational films and documentaries.
There are great deals for free & cheap admision too! Of course check your local library for the free museum pass but also, if your family has Minnesota Care for Health insurance or if anyone receives WIC- you are entitles to a free membership that charges only $1 a person admission.