If I were going to do San Francisco to LA, I'd go on to San Diego, too. Good museums, two great zoos, good scenery on the coast. And while driving down the coast, you might want to make a quick stop to check out the seal rookery just north of Hearst Castle. (And the zebras, other exotics, grazing among the cattle in the Hearst fields along Rt. 1.)

Have never been to Glacier, though it's on my list to do soon, but of all the big/best known western national parks, I'd pick Yellowstone by far over Yosemite, Grand Canyon, Tetons (though you'd be right there at Yellowstone), Death Valley, Olympic, etc. More elk and bison than you can imagine, maybe a moose or two, and it's probably a good time of year to see wolves and bears, too; and the hundreds of geysers keep spouting regardless of the weather. Only problem might be snow/ice if you're not used to driving in it, have a rental car since you can't put chains on them.

Other posters have said you don't need a car in NYC or DC, but I'd go farther and say the last thing in the world you want either of those places is a car (though you might want one for a day trip or two outside DC).

If your heart isn't set on really cold weather the whole time but you want to see some national parks, it's the best time of year for the FL Everglades if your boys would like to see alligators all over the place. Then I'd drive Alligator Alley (a road, not as exciting as its name) to the west coast - Naples, Ft. Myers, and Sanibel Island.

When in SLC, you might want to take a look at some of the national parks just south - Arches, for instance.

I totally agree about the Everglades. They are fantastic. If you do go to Yellowstone, I think it's well worth it go through Grand Teton as well. It's beautiful.