Minnehaha Park - this one is right by our house. We missed the Falls on this trip but found them later in the week by bike.
There is an off-leash dog park through part of the park, which was pretty cool. HOWEVER, I don't think we will be taking our dogs any time soon. The smallest dog there could have eaten either of our puppies for a snack, and they were much better (ie: quieter) behaved than our heathens.