Theodore Roosevelt National Park is one of the places in North Dakota where you want to come back. It is very beautiful in spring and summer. There are several camp grounds and lots of buffalos. If you are not a camp person, you can drive trough the park and enjoy the view. Out of two parks, the North one is my favorite.
The Badlands of North Dakota are beautiful, and it's worth a drive through the park just to see them. We didn't see much wildlife on our drive, though, and didn't have time to camp or hike, so the kids didn't think it was very exciting.
Junior Ranger guides are available that are good at both the North and South Units of the park. Admission of $10 per vehicle is good for 7 days at either site.