Minneopa State Park and Waterfall is a wonderful sight. It is just outside of Mankato, MN. It is one of those waterfalls that you can actually walk around and get into the water.
This is a pretty park in the summer and the winter! The waterfall is big enough to be beautiful and small enough so that it freezes in the winter. The photos you can take here are great.
It is a great place just to relax. You can also have a picnic or a group outing. There are also plenty of hiking trails.