Some suggestions:

It will be hot!hot!hot! in Death Valley (CA), but it's sorta on your way and you can say you've been there. (The pool at the Furnace Creek Inn is divine.) And you'll probably want to pop into Joshua Tree National Park, also.

The national parks in Utah rock: Arches, Zion, Bryce Canyon, Capitol Reef, Canyonlands. If you have to pick one, pick two -- Bryce and Arches (others may disagree). A horseride is a good way to see Bryce. A boat trip on Lake Powell is an easy way to see miles and miles of (flooded, alas) canyons. A float trip on the Colorado River in Moab is another option. (Moab rocks, too!)

If you want to venture a few miles from Monument Valley into southwestern Colorado, Mesa Verde National Park is a unique site.

If you want to make a longer detour from Arizona into western New Mexico, the pawn shops in Gallup are a hoot and a pueblo visit (Zuni and Acoma are the closest) is a must.

I like Route 50 across Nevada -- be sure to hit the slots at the Hotel Nevada in Ely and have a drink at the Owl Club in Eureka.

I like the Moon Guides series for road trips in the American West -- they provide lots of off-the-beaten-path suggestions -- but I doubt that they have one title that covers all the territory you'll be exploring.

Have fun and carry plenty of water!