Tokyo is a haven for streetpass hits.
It’s called “すれちがいMii 広場/hiroba” in Japanese meaning “Passing Mii Plaza”
This is when your 3DS is on in standby mode and picks up other 3DS in the area.
You collect the characters and can unlock maps (if you get people from different areas) also collect puzzle pieces and play various mini-games.
It is a free function built into the 3DS, but there are premium games.
You can also collect these streetpasses in certain games – like Animal Crossing gives you access to a copy of their house and you can buy their furniture or Final Fantasy Theatrhythm unlocks songs to play.