This is a huge project you're starting. Love it, but HUGE!
My personal favorites in each state I'd go to again that haven't been listed:
Alabama
1) Sloss Furnaces, Birmingham - Old, industrial steel plant. Yowzerr!!!
2) Civil Rights Sites, Birmingham - 16th St. Baptist Church, Park, and Institute are all clustered together. We gave an old black guy hanging around the park $20 and he spent an hour walking us around the area giving his perspective of the historic cultural changing events from his youth. You can't get that from a guide book.
Alaska - still on my Bucket List
Arizona
1) Meteor Crater, Winslow - Who doesn't love a big hole in the ground? Sure there's the Grand Canyon a bit farther West, but that took millennia to form. The drive up to the site gives a perspective of how the earth's crust was ripped up in a fraction of a second.
Arkansas
1) Old Downtown, Hot Springs - Too cool to not want to visit it over and over. Besides the roads to and from are some of the best out there.
2) Nearly any secondary road in the N.W. third of the state. If I ever tire of those roads, it'll be time to stop riding on two wheels.
3) Peel Ferry, Peel - First, its free which is reason enough to return, but factor in the ride to and from and I know the bike will be floated across Bull Shoals Lake again and again.
California
1) Pacific Coast Hwy, last road to the left of the map. Has it's views (and traffic). Worth doing again as are some of the routes off of it.
Hard to believe there isn't much else in California that I'd like to see or do again. Will follow this thread with interest to see what others have to say.
Colorado
1) Hanging Lake, East of Glenwood Springs off I-70 - This is a fairly steep trail up to a shallow lake where waterfalls empty over the top of and even through the walls of a canyon. First visited in the early 80's and has become more developed and regulated everytime I've returned. Remains my personal #1 beautiful natural scene within the United States.
2) Just about any secondary road in the Western half of the state. Take your time, look around, make your own discoveries. One of the best meals my wife and I have enjoyed was at a old mission/school in the 4 Corner's area. Don't remember where, but someday I'm going to find that little town again and see if they're still serving lunch to the public.
Time constrains force me to stop here. Great topic Scott, thanks for starting it.