My girlfriend and I spent 4 weeks in the US last year, and 3 of those weeks took place in a car up and down the west coast of California, Oregon and Washington.
Now we are discussing coming back for a coast-to-coast trip. We've been to NYC before, so we plan to start in Boston and head west from there. Must-see destinations for us along the way include Niagara Falls - Chicago - Turtle River, MN (visiting friends) - Mount Rushmore - Yellowstone/Grand Teton. So far the route looks something like this
The massive distances compared to Europe make it a little difficult for us to judge if this is a viable 3 week trip if we also want to see stuff along the way. We're from Denmark, so anything above 200 miles is a mind-blowing distance for us. Also, we're a little unsure what to do after Yellowstone since we have no idea what to do in Idaho/Nevada. We could also go further south through Salt Lake City, but we're still not sure what to see between that and Reno.
Suggestions/comments/advice is more than welcome.
PS: We're also unsure if it is best to drive east-to-west or west-to-east. Its all the same to us, but is there anything we should be aware of for this decision (direction of the sun, seasonal events, etc)?