re: Big Bend - half a day what would you do.

Do you know where you are staying the night before and that night? As some others alluded to, that country is huge so time is a factor in recommendations. Just getting into the park is generally a couple hours from wherever you might be passing through. With only a half day, your best bet might be to just come in from Marathon and drive through towards terlingua. You could do the ross maxwell senic drive to santa elana and then cut back to the main road on whatever that dirt road is called. Stop and look at whatever strikes your fancy. Not sure what the crowds are like now, but when we were there last month the basin was full by around 9 and they were doing one car out/in so plan accordingly if you decide you want to drive up there. If they open the border crossing between now and then, alter your itinerary to allow a full day and go to Boquillas.

Second the recommendation for the drive from Terlingua to Presidio, although the worlds best enchiladas are in Fort Worth. Fight me. There are also some great short dayhikes you can stop and do on this drive. Look up closed canyon and the hoodoos trail in Big Bend Ranch State Park. Do not attempt to drive through BBRSP other than on the highway from Terlingua to Presidio. None of the other roads in the park are paved and google maps will pick up ones that aren't even open to vehicles. Nothing else completely bisects the park.

I'm neutral to meh on Marfa. I certainly wouldn't go out of my way to go through it.