[bcbrown]

I've been to just about every national park in the lower 48. Here's some of the less-well-known ones:

Great Basin has a good hike to the peak, a cave system with guided tours, a nice (summertime) campground, and hiking trails to a really impressive arch.

Mt Lassen has a bunch of hikes, geothermal activity, lakes for swimming, a bunch of campgrounds, and some low-frills cabins.

Arches is always way busier than Canyonlands, but there's also a ton of non-NP features around Moab that are even less trafficked. Natural Bridges National Monument is also great. The Notom-Bullfrom road in Capitol Reef is a beautiful drive, and the campground out there is no-amenity, low-traffic.

Sand Dunes, in Colorado, is super cool, with hundred-foot-high sand dunes, but probably only 1-2 days worth of stuff to do in the park itself.

But if you want beautiful scenery without a lot of people, skip the national parks and look for the national forests. (Or sites in the National Park System that aren't National Parks, like National Monuments etc).

[haswell]

This is a really good point re: National Monuments and Forests. I also found just driving the Pacific Coast highway north of LA county as far north as you can go a different but similarly spectacular experience to many of the parks, with good trailheads and stopping points at various state parks along the way. Obviously a much larger driving commitment, though.