Well, to walk around the trail of Mt. Rushmore, spend time to look at the exhibits, watch the documentary, then to have a meal at the nearby town of Keystone took us nearly the whole day. Perhaps you have time left to drive the scenic 16A.
We were RV'ing, and spent several days in the area, so took it very leisurely. Still, don't see how to do it in a day, unless you are very rushed and just stop to look at something for 15 minutes then drive on. But that is more driving than I care to do.
Besides Mt. Rushmore, Custer State Park, Crazy Horse, we also spent some time in the town of Custer and visited Wind Cave National Park and its namesake cave.
PS. We might have spent nearly a week in the area: 2 days in Wind Cave NP campground, 2 days in an RV park in Custer, and 2 days in a National Forest campground just a few miles from Mt. Rushmore.
"Old age is the most unexpected of all things that can happen to a man" -- Leo Tolstoy