A rental car in Minneapolis is probably the best option for tourists who wish to see more than just the Mall of America or downtown.  However, public transist does exist!  Minneapolis recently built a light-rail system that runs from the airport to both downtown and the Mall of America with 19 total stops.  Buses also run frequently.  A link to maps, schedules, and a trip planner can be found here: http://www.metrotransit.org/ .