If you want to avoid crowds and see some amazing stuff in great weather, go to Canyonlands National Park. The scenery and hiking are spectacular. Of course there will be people there, but it's not what most people would consider "crowded." If the Needles or Island in the Sky districts are still too populated for you, I suppose you could go to the Maze District, which is all dirt roads, 50 miles from nowhere.
There are probably a lot of things to do outside of National Parks and Monuments, but the parks and monuments make good target destinations because of their attractions, and you will get a chance to travel through and explore plenty of amazing non-park lands between those places. Once you get west of Denver (that's a good specification, by the way), give yourself enough travel time between destinations that you plan for more time than just what it takes to drive from A to B at the speed limit. Take extra time and explore. Drive like hell between the East and Denver, though.
Even though I find the western plains attractive, there is nothing that would be missed by going 100+ mph. It's that big.
So here is a short list of some of the Park and Monument places I've been that have good attractions but weren't ridiculously crowded:
Great Sand Dunes National Monument (CO)
Mesa Verde National Park (CO)
Colorado National Monument (CO)
Dinosaur National Monument (CO)
Canyonlands National Park (UT)
Natural Bridges National Monument (UT)
Capitol Reef National Park (UT)
Great Basin National Park (NV)
I also love Death Valley National Park in June, but it seems to be too hot for most
normal humans.
Of course, you already know you'll have to continue all the way to the Pacific to do any beach combing.
As nice as Yosemite is, since you made a point to say you don't like crowded places, I would recommend staying away from that summer circus. If you get off the main highways, you can probably find less-crowded trails, but just the traffic on the roads is going to drive you nuts. Again, this is only assumed because of your statement about not liking crowds.
Maybe there is some hope if you avoid weekends. But, thanks to California's popularity, 30 or 40 gazillion people live within a few hours' drive of Yosemite, so the place is a mess on weekends for sure.
With all due respect to the GC Skywalk, the Native Americans who own it, and those who recommend going there, I think it's a complete rip-off.
http://store.grandcanyonskywalk.com/tickets/
IN ADDITION TO THE [$30] SKYWALK TICKET YOU ARE PURCHASING HERE, all individuals are required to purchase a Legacy pass ($43.05 for adults with all taxes and fees) at the air terminal upon arrival to Grand Canyon West.
http://www.grandcanyonskywalk.com/faqs.htmlCan I bring my camera on the Skywalk?
In an effort to protect the glass walkway and preserve the Grand Canyon, all personal items are not permitted on the Skywalk. This includes cameras, cell phones, strollers, purses, backpacks, etc.
Huh?
So it's $73 PER PERSON to go on that thing, and you can't even BRING YOUR OWN CAMERA ON IT!!!!!. You can take an ENTIRE FAMILY of people, in one vehicle, to Grand Canyon National Park for $25, for an entire WEEK, and it's only $80 for ALL National Parks for the ENTIRE YEAR. And you can bring your own camera and do whatever the hell you want, on your own schedule, 24 hours a day. The views from the rim in GCNP are at least as good as the view from that sky thingy, and the National Park has hundreds of different views to choose from.
I suppose the White Man deserves a good ass reaming after the last 500 years of bad behavior. But $73 per person per day and no camera? That's just ridiculous!