How far outside the park are you looking for things to do? In Minneapolis, there is the Mill City Museum ... however I think it may still be closed. But you can wander the ruins park and walk across the Stone Arch Bridge. There are a few easy trails on the other side along the river. There is also the St. Anthony Falls lock and dam ... I think there might be a visitor center there or a viewing deck. And I think they have paddle boat excursions that go down river and back. Maybe those leave from Boom Island? There is the Sculpture Garden at the Walker Art Center ... kids like the Cherry and Spoon sculpture. Or maybe rent a kayak or peddle boat at Lake Calhoun. I don't live in Minneapolis any more and it's been a while, so not sure what all is still around.
Not sure how long you will be in town and how far you want to venture, but these are some suggestions you can 'google' and see if it might be something you and your son would be interested in.