I'm just seeing this thread now, but I was very pleased to see Na Pali (Kauai) mentioned by several folks, including the OP. I've traveled a lot (about 35 countries, mostly by bicycle), visited all 50 states, most Canadian provinces, and Na Pali is near the top of the list for sheer beauty. I'm somewhat surprised it's "merely" a state park, as opposed to a national park.
I was also pleased to see Sigiriya (in Sri Lanka) mentioned. It's a unique and breathtaking sight, both from a distance as well as on top of the massive rock.
Other favorites of mine:
Redwood Nat. Park. Those of you who only know Muir Woods would be even more blown away by the much larger trees and much more extensive forest in the national park. Similarly the equally impressive redwood forest of Avenue of the Giants, which I believe is state parkland.
Other favorite National Parks: Canyonlands, Grand Canyon, Bryce Canyon, Yellowstone, Olympic, Glacier, Crater Lake, Mt. Rainier, Yosemite
Lake & volcano region of south-central Chile. Hiking up Villarica volcano and looking into the caldera of the active volcano was amazing (and probably dangerous, in retrospect). Also the boat traversing magnificent Lake Todos los Santos, and biking near Osorno volcano.
the forests (both cloud & tropical lowland) of Costa Rica
Parts of the South Island of New Zealand, though the North Island is not too shabby either, and is very different.
Favorite cities in terms of beauty & aesthetics:
Paris
San Francisco
Penang (Malaysia)
Luang Prabang (Laos)
Oaxaca (Mexico)
Guanajuato (Mexico)
Venice (touristy but undeniably beautiful)
Prague