I second what BHopkins said. More about the I-70 route:
Morrison, on the west side of Denver, has Dinosaur Ridge (a short walk to a hillside full of dino tracks) and Red Rocks Amphitheater and Park (amphitheater and some short trails with great views of the foothills and Denver skyline. They may also be drag racing at Bandimere Speedway, between the two. You might not be able to visit Red Rocks if a concert is scheduled that evening.
Farther west of Denver, you can exit at Idaho Springs and drive to the top of Mt. Evans, el. 14,264.
Almost any rest area between Denver and Glenwood Springs will have scenic views.
Colorado National Monument is just off the interstate at Grand Junction. I've never been there, but hear it's similar to the Moab area on a smaller scale.
You must drive Highway 128 from Cisco to Moab if you're in the area. Delicate Arch is something like a 3-mile roundtrip hike once you get into Arches National Park. You may as well spend the day there if you go. The Devil's Garden hike will let you see 4 or 5 arches in a couple hours of hiking.
I just checked out Sego Canyon Petroglyphs on Tripadvisor--they look easily accessible from 70 between Moab and Green River.
You'll probably cut northwest from Green River to Salt Lake. If you like birds, visit Bear River Migratory Bird Refuge on the Great Salt Lake, north of SLC and Ogden. The Golden Spike Historic Site, where the transcontinental railroads met, is out that way in the middle of nowhere. I've always wondered what it's like.
That's where my ideas end. I haven't spent any time west of Yellowstone.
