Anime
Stephanie and I have a spreadsheet for tracking the anime that we own, and we own a lot of anime. It helps us to track what we've already purchased, so we don't accidentally buy it again, and it helps us to track what we've watched, so we can figure out what to watch next. I love watching anime with lots of people, but anime watching parties are annoyingly hard to schedule and to get people to commit to. As a pair, Stephie and I can watch as much as we want, whenever we want; we love a wide variety of genres with a ton of overlap. Even if this was the only thing we enjoyed together, I'd still feel like we were a good match.
But I love that spreadsheet; I love looking over it and seeing what we've watched together. Watching anime is one of the things that we do together as a couple, and I have so many good memories of us: laughing at the ridiculous sexism of He Is My Master in our apartment, or enjoying the egalitarian nature of Haruhi just being "one of the guys" in Ouran High School Host Club, crying over the personal stories in Angel Beats, cheering on Edward and Alphonse Elrich in Fullmetal Alchemist, or fawning over the ginormous tits in Sekirei.
In short, anime is awesome!