I vote for underwater adventures, one of the largest aquariums of it's kind in the U.S., I believe. The "Park at MOA" as it is now called is fun I suppose, but it tends to be expensive and is only real fun for kids. Still walk through it though, cause it is impressive to see. Food courts are located on 3rd floor North and South. Avoid 4th floor at all costs. Theres nothing to see there. It used to be pretty cool, with bars and arcades and stuff, but all the businesses there were taxed to death or just shoved out in an effort to make the place more "family friendly", so all that remains is Hooters and a movie theater. The movie theater is fine, but expensive, and kind of lame considering how cool it ought to be.
Personal experience, I've lived in the Twin Cities since the MOA first opened, and been there quite a bit.