While in the Bid Bend area, be sure to run the River Road... (There are two "River Roads" in Big Bend, one is a dirt road, I'm not talking about that one...)
Basically, from Big Bend take FM 170 west to Presidio, then turn around and go back... The road follows the Rio Grande, so it's got lots of fun twists and turns...
(Disclaimer, I haven't researched how safe that area is lately, I know there's a lot of border violence lately, not sure how in scope the Presidio region is...)
Here's a quickly thrown together route to give you an idea:
http://tinyurl.com/2bn6xmz