Regional forums here may be a better place to ask for specifics.
But in general, it's the same in any city--anywhere there's a big crowd, even better if there will be lots of kids around.
So airport, shopping malls, concerts, sporting events things of that nature.
I don't bother with them much myself. Wouldn't mind more houses in HHA in Animal Crossing to order furniture from, but I don't care at all about the silly Mii Plaza games etc., and haven't seen any other games with street pass features I care about.