For US citizens, are there any locations outside the 50 US states that don't require a passport to fly to and from?
You can re-enter the US if you have proof of citizenship, so that doesn't strictly require a passport -- although it may make your enter procedure more troublesome.
As mentioned above, you can theoretically travel to US territories without a passport, from answers.usa.gov:
Also from that page:
If I'm right that covers Palau, Federated States of Micronesia and the Republic of the Marshall Islands.
However, from visit-palau.com:
so that's out but from visit-micronesia.fm:
So there's another option. I can't find an official page for the Marshall Islands but passportsandvisas.com says you need a passport.
It appears you used to not required a passport for Mexico or Canada but this changed in 2009/2010 and now everyone requires a passport to cross those borders.
Also note, if you're flying airlines may very well insist on a passport or at least make your life difficult if you don't have one. But technically passport-less travel is possible to the places mentioned above.
I'm not sure how things like privately owned islands work -- could you theoretically fly there without a passport? I'd also imagine you might be able to persuade a cruise company to let you helicopter out to their cruise liner and land on that, which might count.