If you're looking for any specific info about FM-170 between Presidio and Big Bend, you should send a PM to
US 41. He was just there three years ago. He posted a
trip report, but the photos have been removed.
My recommendation for a relatively quick hike is Santa Elena Canyon, although
getting to the trailhead will take you south from your route by about 45 minutes. That canyon is so beautiful and serene, it's on my short list of favorite places in the world to visit. You can even wade out into the Río Grande closer to the trailhead and pretend you're an illegal immigrant if you want.

Back in the day, you could pay a guy a few bucks and he'd row you across the river (access south of Castolon) to the village of Santa Elena, Chihuahua, but the government shut that down after 9/11. *sigh*
If you take US 385 north to Fort Stockton it’s pretty bland and you won’t miss much. The only town along the way is Marathon which I thought was edging towards ghost town territory.
It was edging on ghost town territory back when I was last there in the late 90s. Back then, the gas station at the junction still had analog dials. I remember chatting with another driver who tags mountain lions for a living. Heading north from Marathon, the head wind was so strong that we could barely maintain highway speeds in our 88 Camry—didn't even realize for several miles it was still in 4th gear because the wind noise was so loud.