I grew up in El Paso and have been all over the Southwest U.S., but never to the Grand Canyon. I'm planning on that as a vacation.
Some places I recommend:
The Rocky Mountains in Utah. If you can get to Zion National Park, there are rock formations that are phenominal. Reeally, anywhere in Utah mountains is beautiful, though.
Reno/Lake Tahoe/Donner Pass, Nevada/California. I don't know if I-80/US50 is on your planned route, but Lake Tahoe is amazingly gorgeous. If you ski, then all the better. If you gamble, you can do that, too.
If you're going to get further south, Carlsbad Caverns in Carlsbad, NM. Sure, it's just a cave, but when you get into The Big Room, it will take your breath away. It's too huge to describe and the rock formations are natural works of art.
Also, White Sands National Monument near Alamagordo, NM is gorgeous. I don't know if the park is open during the winter, though. Considering the climate in the desert southwest, I don't think it would be closed. It's miles upon miles of pure white gypsum sand dunes. Take the wife, as there is no better place for tanning.
Santa Fe, NM is just a relaxing place to go.
Along I-44 in Vinita, OK (just west of the MO state line) is the World's Largest McDonalds. It's good for a chuckle and you can eat your mcnuggets while traffic rolls by underneath you.
