What you find in the Big South Fork is most trails are either located up on the plateau or down in the river gorge.  There are several trails that link the "high" trails from the "low" trails but very few that go up-down-up-down.

The one area that comes to mind that is steep, challenging and scenic are the trails that connect the O&W railroad bed to the Honey Creek area of the park.  You can park at the Cumberland Valley trailhead (just east of True West on 297) and ride south on the Cumberland Valley Loop Trail (toward Laurel Fork Overlook).  After descending into the gorge, turn left onto the O&W Trail (the old railroad bed) and very quickly turn right onto the Long Ridge Trail at Grooms Branch Creek.  This will take you back up to the top of the plateau on the south side.  The trails on this (south) side of the park are not as well developed or marked as those on the north side, so you either need to be really adventurous, or get a map from Zenith Stables to navigate the trails.  I would say there are more natural features (rock houses, stone arches, caves, etc.) on this side of the park than anywhere else.  The trails are certainly more difficult (less groomed) than those on the north side.  You can return either via the same trails or by taking the Prototype Trail which brings you back to the O&W Trail about 2 miles east of where you ascended on the Long Ridge Trail.  I will warn you, this is an ALL DAY ride.

Alternately, the Park has just developed a day parking lot on the south side of the park (near Honey Creek).  Parking here gives you more time to explore all the natural wonders on this side of the park, but eliminates all the steep, challenging climbs in and out of the gorge.

Another ride would be to park at Gobbler's Knob trailhead, ride east (left) on Gobbler's Knob trail, then turn left onto Hatfield Ridge Trail (you can check out the Charit Creek Overlook Spur while on this trail).  This trail will descend into the gorge and turn left onto Station Camp Creek Trail.  Follow this trail to the river, as this is a beautiful view.  After visiting the river, backtrack slightly on the Station Camp Creek Trail.  Shortly bear left onto Fork Ridge Trail.  Now this trail is STEEP and ROCKY and will take you out of the gorge back up to the plateau.  At the end of this trail, turn right onto Fork Ridge Road for less than 1/4 mile, then bear left onto Charit Creek Horse Trail.  This will descend back into the gorge and bring you to the hostel (an operational hotel in the woods).  After visiting the hostel, take Charit Creek Lodge Connector Trail back up to the plateau, and this will connect you back to Gobbler's Knob Trail.  Turn left onto Gobbler's Knob Trail and ride back to your trailer.  This ride takes 4 to 5 hours, depending on how quickly you ride it.  Even this ride will involve long stretches on flat, wide trails while you are up on the plateau.  The trails down at the bottom of the gorge tend to be more narrow and rockier (but not all).

Hope this gives you a couple ideas for more challenging rides.