The Sierra del Carmen--this is the largest wilderness area in Latin America north of Patagonia. Lies just across the rio from Big Bend NP.
Cuatro Cienegas -- a unique ecosystem, the spring-fed pools that fill the valley of Cuatro Cienegas are host to a number of plant and animal species found nowhere else on earth. Much of the valley is a protected natural reserve, though some of the larger pools (called pozas) are open to the public for snorkeling and swimming.