TheRiver Road*
Location: At the southernmost section of the park, running east to west from near Rio Grande Village and Castolon Village.
Features: This all-day trek of over 50 miles of rough backcountry, 4-wheel drive roads covers a lot of territory that many visitors to the park never see. It allows access to many remote desert trails, historic ruins (including Mariscal Mine), river campsites, and the very impressive Mariscal Canyon Trail.
NOTE: park rangers warn against leaving valuables in your vehicle, as the remoteness of this part of the park invites intrusion by derelicts from across the border.

Ore Terminal Road
Location: In the eastern section of the park, running from near the Rio Grande Village Campground north to Dagger Flats.
Features: This is an excellent route for 4-wheel drive, covering approx. 26 miles, taking about half a day to traverse. You'll pass near Ernst Tenaja. Take the short hike into this amazing, narrow canyon. You'll pass through low grassy area, higher vistas and remnants of ranching areas, as well.
 

Although the park no longer offers horseback tours, there are a few outfitters that offer programs near the park, providing a glimpse into the lifestyle and environment that welcomed the early settlers to the Big Bend. I've mentioned only one on this page, as I know that they offer a broad variety of rides that range from a few hours to several days.

Lajitas Stables (in nearby Study Butte and Lajitas)
For more information, visit their website at: http://www.lajitasstables.com. By the way, I designed their site. Hope you like it.
 

There are several outfitters that offer guided trips throughout the Big Bend area and on the Rio Grande River, including trips that run the three big canyons in Big Bend National Park. I recommend those below for a great time in the desert and on the river. You'll find them in nearby Study Butte and Terlingua.

Big Bend River Tours - http://www.bigbendrivertours.com
Rio Grande Adventures - http://www.riograndeadventures.com