There are over 700 ghost towns in Nevada and I'm sure Arizona and New Mexico have their fair share. I know there are a few web sites that list the locations. See what you can find along your route and check them out. Rent a good SUV for the dirt roads. Some of the web sites will list what type of remains can be found at the ghost towns which can help you select the ones to visit. Sometimes there's nothing left. Old mines can be interesting as well but a little dangerous if you're not careful. People die in old mine shafts all the time, not to mention the other hazards.
Keep an eye out for wild horses, there are over 200,000 in Nevada alone. You will need a good zoom lens, they don't like people.
While I was in Nevada earlier this year I found some of my best shots exploring dirt roads off the beaten path. You can drive for a long time and not see anything but that can make for a great picture as well. Old ranches can have some interesting stuff. If you are venturing up into central Nevada I could give you some good locations to check out.
In Arizona there are some interesting old cliff dwellings that you could visit. Monument Valley on the Navajo Reservation is another great location just be careful with the blowing dust getting into your camera gear.
I find that it's hard to get off the interstate highway when you see something interesting that you may want to shoot. The lonely two lane highways are better for that.
Below is the web site for ghost towns that I used, just select the state at the top and then select the county. I spent most of my time in Lander, Eureka and White Pine counties in Nevada. If you can get over to Death Valley I highly recommend that, it's a few hours West of Las Vegas.
http://www.ghosttowns.com/
