If you're in the area to see Mt. Rushmore, definitely check out Custer State Park. It's well worth the detour (and not very far from Rushmore). We took the Wildlife Loop road - and though we didn't see many bison at first (in fact, we were worried we wouldn't see any), as we drove further along, we saw many, many of these large animals - close to the road and seemingly unconcerned with our car.
While the photos in brochures show green plains, we went in mid-April, a few days after a snowstorm - so, the views were different, but we got some great photos against the stark landscape. We also didn't see a single other car the entire time and when we got out of the car (not near any bison - I do not recommend that), the silence and stillness of the area was amazing. Not sure if that would be the case in the summer as I'm sure there's more traffic.
We arrived at 5:30, just after the Visitor Center was closed (they close at 5pm at this time of year, though the park itself stays open until 11pm), but I really don't feel like we missed anything. However, I would try to be back on the main roads after dark (our Garmin worked throughout the park but when we programmed it to go back to Rapid City, it led us along some twisty and icy dirt roads that are much better navigated in the daylight!). Allow yourself about an hour in the park - that's plenty of time to soak it in.
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