Marfa is a cool place, been too long since I've been.
Of course, you can always look for the Marfa lights, theres a rest area on hwy 90 thats supposed to be an ideal spot to see them. They never appeared while I was there.
Alpine is a cool little city with a few good bars.
Take a drive up to Ft. Davis for sure, you can check to see if the Observatory is open but its a beautiful drive from Alpine to Ft Davis and north of Ft Davis. Theres a reconstruction of the old fort there if you're into that and a very cool old hotel from the late 1800's thats still open.
Definately make the drive down to Big Bend while your there even if you dont spend time in the park itself.
A very cool drive is to take 67 south from Marfa all the way down to the border in Presidio. Then head east along the river road that follows the Rio Grande through Terlingua and catch 118 north back to Alpine and 90 west back to Marfa. That is a beautiful drive, river road runs through Big Bend State Park and the whole trip is just outstanding views.
If you're not hiking in Big Bend itself a drive through is pretty amazing too but I think you'll end up paying for a park pass even if you just want to drive it.
Lots of cool stuff, but its BIG country out there. When you see a town on a map, remember there is NOTHING in between one town and the next... Carry a gallon of water and snacks in the car everywhere you go and dont run the tank down to empty. People die out there when the car breaks down and they try to walk out.
Also if you've never been, you will learn fast that everything out there sticks stings pokes bites hooks or cuts. The flora fights back in the desert ! If you're doing much hiking, a pair of tweezers will make pulling cactus spines out of you much more pleasant.
That region is the most beautiful I have seen anywhere, I hope you have a good time.
Where are you guys going to be hiking ?