I get a lot of streetpasses at the mall, I mean not a lot, but 1-2 each time I go it seems, on a Saturday or Sunday or other times when it is really busy. I was thinking at a mall you could also catch workers who were on break playing as well as people who are playing in the mall. This mall does not have stores with demo units so its not that. I also got one on the highway or thruway in NY state as we call it, I assume I got one on the highway as I left my house and did not stop driving and I checked the DS when I stopped at my first stop and there was the streetpass.
I always make sure to walk around the food court every time I go to the mall, the 3DS is hidden in my purse so no one knows what I am doing. I would imagine if someone caught me trying to get streetpasses that would make me look quite creepy if you get my drift. In fact I probably shouldn't even be talking about this here. You should even walk through the food court several times during your mall trips if you stay for a while.
I am sure the dentist's office would be a great place to catch streetpasses, when I was going to the dentist for braces there would always be a ton of kids in there playing some kind of game system (although this was quite a few years ago), so I would imagine it would be the same for the dentist in your area. The dentists offices that I have to visit with my family don't really have any kids in them at all.
You aren't allowed to smoke outside of public buildings such as medical buildings in NY state, you aren't even allowed to smoke anywhere near a doctors office or hospital campus not even in your car so that would not work.
Also going by schools does not work, I live directly across from a school, like one fairly narrow street inbetween my house and the school and nothing. Kids aren't allowed to bring electronics to school that is probably why.
We also need a friend code thread for the 3DS, I could not find one (there is one for the Wii U), and I have no friends.