jimh009 - 9:56 am on Jan 8, 2011 (gmt 0)
It really depends on how adventurous you want to be, and what you want to do (do some hiking, just driving around seeing the sights, stay on paved roads or head off onto the gravel). So, without knowing that, it's kind of hard to make some definitive suggestions.
That disclaimer aside, here's a few spots you'll want to look up on the map that might be of some interest to you on your wonderfully long road trip. I've traveled extensively throughout the West (on and off road), so am familiar with lots of places most tourists zoom right by.
1. Sawtooth Mountains in Idaho. Endless stuff to do here with beautiful views.
2. Driving the Beartooth Highway between Red Lodge, MT the outskirts of Yellowstone Park. It's an absolutely gorgeous drive, and the traffic isn't too terribly horrendous. Open in summer/early fall only.
3. Mountains of Central Nevada. If you'll be visiting Great Basin Nat'l Park, be a bummer to zoom right by the mountains of Central Nevada (south of Austin). The mountains of Central Nevada are one of the loneliest places around, and are an ideal place to explore if you have a 4 wheel drive, high-clearance vehicle.
4. Crater Lake in Oregon. It's just so different, you got to see it!
5. Driving the Coastal Highway in Northern California and up through Oregon. Lots of traffic, but gorgeous views of the Pacific Ocean. The open use of Oregon beaches is great, too. Don't forget about the Olympic Peninsula, either.
6. Rocky Mountain National Park. You really weren't planning on bypassing this, were you?
7. Hiking in Glacier Park. Hard to beat the day hiking of Glacier National Park. When visiting Glacier, get out of the car and hike the Highline Trail, Iceberg Lake Trail or the Grinnell Glacier Trail - or best of all, do all three. Just watch out for the bears.
8. Devils Tower in Wyoming. If you're visiting the Badlands, it isn't a huge detour to go see this. Who knows, maybe you'll have a "Close Encounter" while visiting it, too.
Hmm...I can go on for hours on this topic, but hopefully this gives you a few more ideas.