There are tons of trails here in South Dakota and all around Mt. Rushmore. It will be extremely busy here during that time period though, just a heads up. Keystone has a ton of museums and attractions for kids, and it is less than 5 minutes from Mt. Rushmore. I would also suggest checking out Custer State Park and heading up to Deadwood using the back roads. Then you can just jump on the interstate and hit devils tower along the way. Devils Tower is about a 3-4 hour drive from South Dakota, but there are a couple things to check out along the route. Their is a buffalo jump site right off the interstate in Wyoming. Let me know if you have any other questions regarding South Dakota since I live in Rapid City!