Fun stuff to do in minnesota that is cheap?

Im having the car this weekend and going to hang with my younger bros and sister. They are 13,14 and 16. Im willing to drive over by maple grove, brookyln center/park and by mall of america. I dont really know my way downtown. i only know how to get to the place with the big spoon and cherry. Do you know any fun cheap things we can do? maybe even free?