These are memorable in my mind:
Glacier N.P. -- Highline trail: Logan Pass to Granite Park Chalet, down to Swiftcurrent Lk/Many Glacier campground.
Yosemite's Four-Mile Trail to Glacier Point, then the Panorama Trail back to the Valley.
I've done several trans-Sierra day hikes, but that requires coordinating with another party going the opposite direction, and swapping cars.
If you haven't climbed Mt Shasta, it can be done in a long day.
Zion N.P. -- Narrows hike, top-to-bottom.
