What: Death Valley's Badwater, a salt-encrusted old lake bed between forbidding mountains, is the lowest point in North America. It's especially vivid and perplexing just after sunset when there's a prominent moon -- because how can there be a moon in the sky, when you seem to be standing on its surface already? You'll be sorely tempted to pose for an album cover photo.
Also, be sure to hike a few hundred yards out onto the salty flats, so that you can look up at the east canyon wall for the white sign that's 282 feet above you. That's where sea level is.
Why: Nothing says San Francisco more clearly than a cable car climbing a hill.
What: The cable cars of San Francisco cover three routes: Powell-Hyde, Powell-Mason and California Street. You want a Powell-Hyde ride, and you want to begin at the turnaround at Powell and Market streets, near Union Square (where there's often a queue, along with buskers and beggars).
Also, you want pole position, standing on a running board with a hand-hold on the pole at the right front of the car (unless, of course, you have the opportunity to give a bright-eyed kid that spot).