You'll be glad you did. We were there for five nights in August and it makes a great base for the many things to see around it.
Go to the historic Game Lodge and look at the fireplaces. Made from crystals brought up from Wind Cave and Jewel Cave, the bartender pointed out to us.
We didn't need to take a buffalo safari - they came to the campground every day, a big herd. You can walk up to the fence and get some amazing close up photos.
Be sure to see the Sylvan Lake - beautiful. Overall, one of the best state parks I've ever seen and a terrific vacation location.