Visiting the state parks and hiking around is an absolute must, especially if you enjoy nature. Bring your camera! Gooseberry Falls State Park is the most visited state park in Minnesota. It has five waterfalls and a number of trails along with an interpretive center and gift shop. It is also handicapped accessible.
Split Rock Lighthouse is also something to put on the top of your list. I believe the admission charge is around $6 but well worth the money. The lighthouse sits atop a 130 foot cliff. You can see exhibits, watch a film about the history and building of the lighthouse, and take a tour. Go to the top of the lighthouse and see how it works! Then take the trail down to the shore (many steps) and take pictures.
There are many waterfalls that cross underneath Hwy. 61, with wayside parking. Stop the car and take a look! One of the neatest that crosses under the road is in Temperance River State Park, with potholes, cauldrons, cascades, and sheer rocky cliffs. Also check out the Cross River from both sides of the bridge on the Hwy. You can park and walk down the steps to get a closer look.
Judge C.R. Magney State Park has the most bizarre waterfall anyone has ever seen, called Devil's Kettle. For those of us not on a regular exercise program it is a somewhat difficult hike to get to the falls because of the many steps to go down and back up. But otherwise it's a nice walk through the woods high above the river. It is definitely worth the effort in the end if you don't have a heart attack on the stairs. Anyway, half the river disappears into the ground, never to be seen again, and no one, not even geologists, can explain it. So don't fall into the river because you'll disappear forever if you wind up in the kettle! Must see to understand what I mean. As we were leaving the park we picked up a flier explaining the different theories regarding where the water goes in Devil's Kettle, which was interesting.
Tettagouche State Park also has a somewhat long trail compared to the others (maybe two miles round trip) to see the falls. The trail to see the lower falls has quite a few steps. The falls are beautiful, of course, but no more beautiful than many of the other falls we saw. So unless you're up for a walk in the woods and have the time, put these falls last on your list.
In some of the parks the trails would be long and difficult for older people or very young children and potentially dangerous, so pick and choose carefully where to go. Gooseberry Falls State Park and the parks where the Falls are close to the road or parking lot are the best bet. Grand Portage State Park has a somewhat long walk to get to the high falls, but it is handicapped accessible and easy.