Okay, since you know Tucson, I am starting from there.

In New Mexico, you will come through Las Cruces. There is Mesilla, an old Spanish town & plaza where Billy the Kid used to drink and socialize.

http://www.mesilla.com/

An hour eastwards, you find White Sands Natl. Mon. which is a most outstanding desert experience.

http://www.nps.gov/whsa/

Just a little further is Alamogordo with its Space Museum:

http://www.nmspacemuseum.org/

Further eastwards, if you have a sense of humour, you may do a little detour northwards to Roswell, where you find a strange UFO museum:

www.roswellufomuseum.com

Southwards, you come to Carlsbad with a famous cave (I haven't been into the cave):

http://www.nps.gov/cave/

Otherwise, this part of New Mexico resembles the scenery in the Wall-E movie.

If you drive southwards, take the road through the mountains and Fort Davis in order to catch some different scenery after driving through barren arid country. Fort Davis is a Historic Site and a nice little town:

http://www.nps.gov/foda

From Fort Davis, proceed to Marfa and then the US 67 to Presidio and from there the 170 eastwards - it is a very scenic road along the Rio Grande.

Before you reach Big Bend Natl. Parkpark, stop in Terlingua - a most unique ghost town with a gorgeous bar (Starlight Café) and a museum-like general store:

http://www.historic-terlingua.com

Big Bend is a very attractive Natl. Park with several campgrounds. You find desert, mountains and the banks of the Rio Grande river (opportunities for canoeing).

http://www.nps.gov/bibe/

From Big Bend, you should take the US-90 to San Antonio to get a proper impression of rural Texas.

Good suggestions above.

I'm assuming you're staying generally within the I10 "corridor". Definitely recommend Carlsbad Caverns, especially it's Big Room which comprises 8+ acres and its ceiling is 350' high. Don't miss the bats at sunset.

Also Monahans Sandhills State Park (TX) with its large sand dunes.

And of course there's the Hill Country northwest of San Antonio.

Big Bend is certainly worth a visit - just remember it will be extremely hot in Aug so take heed of precautions re water, etc.

In the Ft Davis area, visit the McDonald Observatory. At almost 7,000 ft, the views of the stars can be amazing.

Ft Davis will take you near Marfa and Alpine on US 90, very pleasant known for artist colonies and with their elevation the cooler temps at night will be welcome in Aug.

Wow you guys are awesome. Thank you so much!!! Great ideas!
Karn

if you are going to be in Alamogordo, take a detour and go to cloudcroft. it will prob 35 degrees cooler, and the scenery is beautiful. maybe you can plan to spend the night there, that would be a nice stopover, and not really out of the way.