Fly an airplane. Make a habit of sending cards.
I actually flew the airplane about a year ago, but completely spaced on crossing it off the list. Here's proof:
As for cards, well, it's hard to establish when one can cross "make a habit" of something off a list, isn't it? Perhaps I should keep that in mind for my next list and really focus on concrete actions. But I've mailed more cards this year than I did last, and more last than the year before (and that was the year I got married and mailed thank-you cards like it was my job), so I'm comfortable calling this one done and done. It's a habit I really like; there's something personal and exciting about sending a handwritten letter, sometimes with a picture or a newspaper clipping tucked inside, and thinking of it showing up on someone's doorstep in a mix of junk mail, catalogs, and bills.
Well, that and I really like shopping for stationery.