If you are driving through South Dakota on the way to the Mount Rushmore area, a detour to Badlands National Park is a must! For directional purposes, put in Ben Reifel Visitor Center. Before you reach this large visitor center, you will see some pull offs which make for excellent photo stops!
Then just before the Visitor center, a busy parking lot that marks the start of the Window and Door Trails. They are both very short trails, the door trail is about 3/4 mile round trip. The Door trail officially ends at the end of a boardwalk, but you can actually continue on beyond and walk among the rock formations. The window trail is only 1/4 mile round trip. And there is a lot to see right from the parking lot without walking at all.
After you have explored this area, drive a little further to Ben Reifel visitor center. There are some cool exhibits here including different fossils found in the area. The kids can also get stamps for their books here.
After you have stopped at the visitor center, continue on to drive through the 30 miles of rock formation. For map purposes, get directions to Pinnacles Overlook and then back to the highway. This detour will add about an hour of driving time to your trip, plus however much time you choose to spend in the park.