M.B.
What about Tucson? We went almost 2 years ago. I was pregnant and our daughter was almost 18 months. We went up Sabino Canyon. There is a "train" that you can ride through the canyon and see the sights. They have a visitor's center with info about the canyon and a few informative movies. It was very enjoyable, even with a little one so young. I am sure that your children would get more out of it since they are older and your parents would enjoy seeing some of the sights of the desert. There was also the Titan Missle Silo which was very interesting. It might be a little hard for your dad if he has a hard time walking as there are a lot of stairs (they may have an elevator, I am not sure since we didn't use it), but your teenager would love it! I am sure there are some other things that I don't know about (look online and see if there are websites about things to do), but hope that gives you a good start for maybe going down there.