Bison are the most easily spotted and largely abundant through Wind Cave. In the many times I have visited Wind Cave, I never failed to spot at least a few bison. Like Custer State Park, immediately adjacent to the north, you can spot these Bison along the main park roads, Highway 385 and SD 87 in Wind Cave.
For a more secluded experience in getting closer to nature while in the safety of your car, take NP 5, at the northern boundary of the park to head into the backcountry of the park. On these dirt roads, you are likely to get a much better view of these majestic animals. The roads are not to rough, but do not go exploring with a low clearance vehicle. Keep your eyes peeled as the bison can blend in. Be sure to not get to close or make noises that might agitate the animals, they might look slow, but can run up to 35 mph. Exploring the backcountry of the park from your car is an easy way to get closer to nature away from the already low crowds at Wind Cave.
Bison can also be seen from the trails at Wind Cave. This is more dangerous then spotting them from a distance or in your car. Many of the trails in Wind Cave are game trails that were made by bison, it is not uncommon to see them in a field grazing on grass. If you see a bison in close proximity, be quite and give it a wide birth, this will save you from any problems. Bison are incredible animals, Wind Cave National Park is a great place to see them in their natural environment.