For a nice short bike ride, bring a bathing suit or a fishing pole and ride the Grace Coolidge Walk-In Fishing Area Trail. The trail can be accessed from the north near the Center Lake swimming beach, or from the south from the parking lot of the Grace Coolidge Campground on the north side of Highway 16A. The trail runs between the Grace Coolidge Campground and the Center Lake Campground [1], a distance of 2.8 miles, and follows Grace Coolidge Creek the entire distance.
The trail is moderately difficult. From the south, the trail parallels the creek, with several crossings along the way. Be careful as the crossings can be challenging, especially in the spring when stream flow is high. This is also popular creek for trout fishing so try not to create too much mud when splashing through the water.
As you near Center Lake, the trail begins to climb sharply. From the top of the hill you can see the lake nestled in the pines. The descent is steep and will bring you to the swimming beach near the shower house at the lake.
Links:
[1] http://www.moon.com/destinations/mt-rushmore-the-black-hills/the-southern-hills/custer-state-park/campgrounds