The Big Bend
I spent two weeks at the end of 2007 (Dec 19 to Jan 2) in west Texas, flying through El Paso, which is about 300 miles northwest of the area. I spent most of the time in Big Bend National Park, which is named after a major bend in the Rio Grande. I also visited a nearby state park and the Davis Mountains in the middle of west Texas.
My rental car started as a PT Cruiser. Part way through it, the engine lights came on, all the dash lights started flashing, and the thing was beeping at me constantly. To my surprise and pleasure, Dollar Car Rental towed a replacement car 300 miles to me, and took the PT Cruiser back. So I had a Dodge Avenger to finish the trip with.
I chose west Texas because I was hoping for warmer weather at this time of year. Most days were warm enough to hike in a short sleeve shirt, but most nights it got cold at sunset, which was at 6pm. A number of nights were well below freezing.