My co-worker Tammy, along with her family and some friends, recently spent a long weekend at the Little Missouri State Park (northwest of Killdeer) and enjoyed superb trail riding. You do bring your own horses here, and what you'll find are excellent facilities for them with camping right next to your horses.
Little Missouri State Park offers 30-miles of scenic trails. Trail maps are available at camp headquarters when you check in. And you'll find the trails are marked like downhill ski slopes (so you can ride to your level of experience).
Tammy's crew enjoyed perfect weather, a lot of riding and nights 'round the campfire and under the stars - telling jokes and making s'mores.
Tammy also brought along a video camera to capture the scenery and riding experience. Watch for yourself!