There is a little coffee shop near a major tourist intersections that I used to sit and watch people. so you got a good mix of locals, tourists, holes and druggies all mingling together. Families that think they are in the big city are my favorite because the excitement and caution mingled together.
I find a nice busy bar to be another good one. local bars with regulars. people are so happy to see each other, make new friends. Everyone is looking to get laid but only a few have the confidence to really go for it.
I don't really enjoy Walmart because there is a certain level of desperation and despair from so many people that I don't find enjoyment in, yet, it has its moments.
The city busses are interesting. But while trying to not make eye contact and keep to myself the people watching is awkward, but one does see a lot all the same.