Attend a once-in-a-lifetime event
It can be the Olympics, the World Cup, the Super Bowl, the World Series, Mardi Gras, Carnival in Rio, Oktoberfest (in September) in Munich, Burning Man, whatever. Granted most of these events happen once a year (obviously, not the Olympics), but the expense of attending most of them as well as the simple time and life constraints means that for most people, going once is once-in-a-lifetime.
Have a one-night stand in a foreign country
As promised: doing sex! Why in a foreign country? Um, why NOT in a foreign country? Sure, you can, and probably will, also do this within the continental United States, but a steamy one-night romance in an exotic foreign land -- like Canada! -- is just one of those life bucket list items. Because it just is.