I went to an anime/gaming convention a week ago, and oh my god, Streetpass, specifically Find Mii, is incredible. Waiting in lines, or doing something you didn't personally want to do, is perfect for compulsively checking for that greenlight indicating that someone walked past you, and you now have a free warrior in Find Mii. Whats better is trying to figure out who exactly it was (everyone being dressed up as Zack and Pyramid Head didn't make it easy).
Someone even said "Hey, guy... if you go past room 143 (this took place in a large hotel) you'll get a street pass."
It's like those facebook games I love to hate on, except it allows me to maintain my high horse stance because I can say "I'm actually seeking people out in person."
Basically, not knowing what to do became, "lets go to an area we haven't really covered in search of new people." It was great. It doesn't sound like much, but it was a big part of my experience at the con (nevermind taking pictures and keeping track of my spending with the notepad feature). Next time, I may add a message like "free beer in room <whatever my room is>".
I even beat someone in SSFIV's figurine battle... I spent a few minutes guessing at what would make a good team, and apparently it paid off. Watching the battle unfold (which happens with no input from you besides you forming the "deck") was like cheering for a football game or something, and I knew that one of these Tifas or Helghast walking around just got owned by me.
I have a fleet of 35 Miis that I encountered throughout the weekend, and when I go back next year, I'm sure a decent number of them will return, increasing their levels even further.
It really snuck up on me, and was really cool.