The topography here may make you forget you're in Nebraska. Some of the hills are dramatic and there are bluffs and canyons everywhere. Get a trail map at the park HQ and look for the big loop you can do around the NW quadrant of the park. One of the rangers is an avid biker, so you're in luck if she's manning the desk. I did the loop counterclockwise, but it may be just as good clockwise. There are a couple drawbacks. The entrances to the trail from the road don't match the map well, so you may spend some time looking. The northeast end of the loop is simply mowed prairie grass, even when it's in the pines, so it's not that interesting. Also, the sand hills are subject to rapid erosion, and the worst of the grades have no waterbars or armoring, so they are deply rutted. However, the western edge of the loop is very good. This trail is worth a stop if you're in the area--unless you're continuing on to the Black Hills, less than two hours away--save you're energy for the excellent singletrack there.