A couple of thoughts, first it is HOT in west Texas at the moment, so perhaps something indoors, or during the cooler evening hours, like the West of the Pecos museum in Pecos Texas. Fort Davis in the Davis mountains is about 160 miles south of Carlsbad and is always good for a couple of days, there is McDonald Observatory about 15 miles west of Ft Davis, with a nice visitors center and public observing star parties 3 or 4 nights per week, for outdoor activities there is also some nice walking trails at the Nature center about 2 miles out of town http://www.cdri.org/nature-center.html and of course there is Marfa TX about 30 miles farther south which has became an artists mecca over the last 15-20 years.
If you do opt to go to Fort Davis, take hwy 17 (or 118 if you are adventuresome and in a smaller coach) vs 67 as they are far more scenic though have lower speed limits (most of 67 is a 75 mph speed limit) I have never driven hwy 118 from Van Horn to McDonald Observatory in anything larger than a 21 ft camper van, but from memory can't think of any issues, it is somewhat narrow, but very very little traffic, and a number of pull off picnic table sites along the way.
