This weekend, I took an excellent jeep tour through Far Flung Outdoor Center in Terlingua. Our guide, Elizabeth, took us on a thoroughly enjoyable three hour tour, telling us about the geology, botany, zoology, history, and prehistory of the area. This would be a great activity for a family, or a group with a variety of fitness levels.
Visiting the Barton Warnock Environmental Education Center near Lajitas is another excellent way to learn about the Big Bend area before going to the National Park or the Big Bend Ranch State Park. The Center presents the archaeological, historical, and natural history profile of the Big Bend area, plus it has a gift and bookshop and desert garden. You can also buy camping and river use permits and licenses for the state park in the Center's administrative offices.