Custer State Park is a great escape from the crowds at Mt Rushmore and Rapid City - nice place for a hike, picnic, drive, etc.
Evan's Plunge is on the way to Rapid City if your kids like to swim. It's nothing special, but I liked the fact that it seemed to be frozen in time - sort of a relic from many decades ago.
There's a boatload of touristy things in Rapid City, including various reptile parks and a drive through animal sanctuary (buffalo, mountain goats, big cats, etc.) that my kids enjoyed.
1976 FJ40 - green and mostly stock
1996 FZJ80 - not so stock