You will need to go into the actual streetpass plaza to get more than 10 streetpasses outside of PAX (when you hit the maximum, you won't streetpass any more until you clear it out), but you don't need to do anything special within AC. I have three 3DSes and I have the silver streetpass badge on all three. I almost never go into the HHS and I don't think I've ever gone into the HHS with my one town. I got the badges just the same. I see the green light, open up the 3DS to play normally, and when I'm done playing, there will eventually be another green light.
Just to clarify, I don't go into the streetpass plaza myself. Again, I don't think I've *ever* been in it with my one 3DS. You only need to do that to keep getting streetpasses at PAX, since it will be unique people filling up your space. I'm actually no expert on that, but I can just reiterate that I've gotten up to silver without doing anything but having my 3DSes in close proximity all the time.
(by my calculation, I should hit gold around Halloween)