Honing your strategy and watching your Knight duel against StreetPassed foes is awesome.
http://www.nintendoworldreport.com/editorial/37966/shovel-knight-might-have-the-best-use-of-streetpass
The concept for StreetPassing in Shovel Knight always seemed really interesting. Inspired by games such as Mario Bros., it works by recording three rounds of your own Shovel Knight making movements around map collecting gems and trying to defeat an invisible foe. When you send that recording out to the world, you swap it with other people's recordings, and then your pre-recorded characters duke it out in a best-of-three match.
Fortunately, I got to experience a ton of Shovel Knight's StreetPass Arena firsthand at TooManyGames 2014. It was amazing to watch, making me overly enthuiastic for StreetPass for the first time since last year's introduction of the new StreetPass games. As you quickly learn when you get your first StreetPasses, your strategy might be dumb. I learned that, not knowing I could even magic in the StreetPass matches. My little Knight, jumping around stabbing the air and trying to collect gems, had a pitiful debut.

Then I went back to the drawing board. I realized people linger slightly in the opening of these matches, so I sent off a quick fireball to open up every match, which turned out to be a devastating manuever. Some folks were wise to that trick, though, and would jump over it or use the Phase Locket. Then, it's off to the races as you hope your recorded Knight stabs the air at the right time to defeat the other player or collect the more gems before time runs out. It's exhilirating to watch because while you might know your exact movements, you have no idea what the rival player is going to do. It's a tense competition that doesn't have real-time interaction but is still fascinating to watch.
Over the course of my two days at TooManyGames, I watched more than 30 StreetPass duels, tweaking my strategy after each batch I studied. While, thanks to the unfortunate difficulty of getting regular StreetPasses, I don't expect to be embroiled in Shovel Knight duels all the time, I'm looking forward to the PAXs and the New York Comic-Cons of the world where I can match up against other players, tweaking my strategy as I go.
Unlike most of my games I have active for StreetPassing, I will go back to Shovel Knight's because it's something fun to watch that, even when you're finished the game, is still compelling.