When? What do you enjoy seeing/doing?

Need more info before giving any real detailed advice. But I would cross Washington and Oregon off your list for sure, if you want to visit those states fly into Seattle or Portland.

Perhaps Yellowstone? There is a lot to see and do in Colorado as well, plus you aren't far from Utah if you get to SW Colorado.

Take I-70 west into Utah and turn left on 191. That will get you pointed at the red rock country. Off the top of my head, I would recommend Arches National Monument (Moab), CanyonlandsNational Park, Zion National Park, Bryce Canyon, Canyon DeChelly, Zion....the mind boggles. Get a good map of Utah or do some Googling. A few moments using Google Earth and you will fall in love. After you have seen as much red rock as you can handle, take I-15 back up to I-70 and onward to Denver.

This query is a little bit wide-open for what we typically address here on Fodors, but I'll give it a whirl:

The first thing one should think about, within hailing distance of Denver, is Yellowstone N.P., and if one heads in that direction the scenery of western Montana seems to beckon nearby.

No doubt that another major attraction to the west is The Grand Canyon, and while awkward, one can make use of Interstate 15 for a fairly straight-ish trip from Butte, Montana to St. George, Utah, where you might cut off to go to the Grand Canyon.

Don't know how (hell bent, sometimes...) you are about getting to the actual Pacific Coast, but IF that isn't a priority you might enjoy a circuitous route connecting Denver-Yellowstone-Western Montana-Eastern Idaho-Salt Lake City-Grand Canyon-New Mexico(???)-Colorado Rockies-Denver.

Guessing:

Night #1 - central Wyoming?
Day 2 - arrive at Yellowstone
Night #2 - Yellowstone
Day 3 - explore Yellowstone
Night #3 - Helena, Montana???
Day 4 - drive around the MT rockies
Night #4 - Idaho Falls, ID area ???
Day 5 - Drive to Salt Lake City
Night #5 - Salt Lake City, UT
Day 6 - bit of exploring SLC - then driving on
Night #6 - St. George, UT area ??
Day 7 - arrive at Grand Canyon
Night #7 - Grand Canyon area
Day 8 - explore (more of?) the Grand Canyon
Night #8 - Flagstaff, AZ ???
Day 9 - drive to New Mexico
Night #9 - Santa Fe or Taos, NM
Day 10 - explore Santa Fe/Taos
Night #10 - southwest Colorado
Day 11 - mountain drive to Denver
Night #11 - Denver
Day 12 - Fly home


Now IF, say, you live in Vermont, and your idea of "the west" has long been the Hollywood Walk of Fame, and you've never been, and if you long to put your feet in the Pacific Ocean (for the first time) then obviously you wanted more from such a trip.

Outside of that, the outline I made there might give you a backbone of something to consider.

(just guessing...)

Here's our whirlwind plan with about your amount of days traveling (before we settle down in Breckenridge for a bit). We leave Friday. (!!!)

arrive in Denver, then...
Cody, WY
West Yellowstone
Old Faithful Lodge
Grand Tetons
Panguitch (Bryce)
Grand Canyon south rim
Cameron, Az
Moab, UT
Breckenridge
back to Denver

We will see Yellowstone, Grand Tetons, Bryce,(maybe Zion), Grand Canyon, Arches, Canyonlands, Rocky Mtn NP and amazing scenery along the way. Most people will hate our trip. It's going to be work, but we are going to see so much. I wouldn't have dared mention it until I saw NorthwestMale's potential itinerary include Yellowstone and Grand Canyon in the same trip. (unheard of here on Fodors!)

Awesome! Thank you everyone! I plan to go in Mid-Sept and I would love to see the major natural sites and key historical cities/capitals...

The most I can say is "inside of car doors" trips. Whatever floats your boat.
One plus may be that in September the OP might be able to travel with minimal advance hotel reservations.

With 10-12 days, I would focus on Colorado and Utah.

Thanks Happytrails.See and enjoy things, and not do a driveby.

Mid-Sept? Like in 2 weeks or next year??

Sept and October are prime times for visits to the West/Southwest. Don't count on staying in any NP at this late date if it's for this year.