We planned to spend 2 nights at Badlands National Park. We got to Badlands National Park in the late afternoon and drove to Sage Creek Campground. It is free to camp at Sage Creek and is a first-come, first-served campground with. (NOTE: Although it's free to camp at Sage Creek Campground, it is $30 for park admission unless you have an annual pass). It's important to know that Sage Creek Rim Road is dirt and you are not supposed to take a vehicle more than 18' in length down this road (although this does not apply to horse trailers). When you arrive, you can find an empty campsite by a numbered post. Then you write down your name, length of stay, and site number in the reservation book. Many places recommend that you leave something in your camping spot to make it easily visible that it's reserved when you are not there. We use two cheap folding camp chairs for this. Ours have a small hole on each foot we put a tent stake through. Since it was super windy so we used two (one on the front and one on the back on opposite sides).
There are also some extra parking spaces and a large area in the center where someone could pitch a tent. Each marked spot has a picnic table, some of which have covers. There is no water available at this campsite, although there is a bottle filler available at the visitor's center (at the other end of the park). There is also two pit toilets at this campsite. If you navigate directly to Sage Creek Campground it's faster because you go to the next exit on the interstate than going through the entire park. If you're coming in after the park closes this is probably the best way. We did this and then drove out of the park and back in to purchase our annual park pass the next morning.
One of the things we love about Sage Creek Campground is that there are often buffalo near the camp (or even walking through!). This trip there were lots of prairie dogs in the camping area. The scenery is also amazing in this area. One of the interesting things about Badlands National Park is that it is an "open" park, meaning that you can walk off trail in the park, which makes the Sage Creek Campground a great place to spend time exploring the Sage Creek Wilderness Area. There are also many established trails throughout the park also. We spent the evening of day 3 in our campsite and Jerry took the opportunity to walk up a trail on top of a nearby hill. The views were amazing. In the picture below you can see Jerry's silhouette on top of the hill.
We accidently visited during a major heatwave. When we first got to camp it was about 105 with high winds. I felt like I was in a convection oven. It's important to make sure that you drink a lot of water and that your pets either have constant access to water or are frequently offered water. Even though it was hot, shortly after dark it cooled down to the point that by morning we were using a light blanket.