Big Brother.
Among other things, DesertWife and I attended the 1986 World’s Fair in Vancouver. It was August and hot and humid, especially for Vancouver. I remarked, “Funny how cranky is cranky no matter what the language.”
There is this park between the Blue Mosque and Hagia Sophia with benches. You can see both sites from there. There’s a park area that has kids playing. It’s (obviously) a tourist area, so there are tourists from all over the world doing the tourist thing. (really, all over. there were people from everywhere there when I was touristing) It’s close enough to the non-tourist routes so there are non-tourists going about their normal daily life. (there are people using the mosque to worship in - not just to take selfies near) There’s street food so you can get something to snack on and eat while just enjoying the people.
And the view is insane.
Bookstores and malls. I have this weird sort of thing where I enjoy watching people read, or shop, or do something totally unself-consciously. It’s endlessly fascinating and very soothing. Supposedly, there is a YouTube video genre of people who film themselves studying - I’ve searched, but I can’t actually find any such videos.
Plus there’s the intellectual satisfaction of seeing someone, especially a child, completely immersed in a book. I occasionally see a kid walking down the street with her nose in a book and think, “There goes my childhood”.
OMG…I did that, too!
Any beach on the Nevada side of Tahoe. You can sit and drink beer.
+1 on airports. A friends swears by the Oklahoma State Fair.
One of the best parts of visiting Las Vegas is strolling around with a beer to take in the world class people watching.
Either of the casino strips in Las Vegas would probably top my list. A bit cliche, but Walmart has some world class people watching as does most any Waffle House, especially between midnight and 4 AM.
ETA: Times square in NYC, but from a second story window of an air conditioned room to eliminate the odors and improve air quality.
Magic Kingdom hands down.
There’s a good blend of “what would you like to represent in your clothing” mixed with a whole lot of “why in the hell would you wear that”.
Lots of colleges and sports teams for the former, long-sleeved black cotton shirts and jeans on a hot summer’s day for the latter…interesting as can be.
Vegas, baby. With Venice Beach as a runner up.
Almost any state or county fair. The odors can be a bit problematic, but still…
The next best thing: Mon Ami Gabi Las Vegas.
Best for breakfast so you can see the various Walks of Shame. 
Sidewalk terraces in Spain. Of course they’re likely to be more interesting in a big town than in a medium-sized one, but for a townie, sitting down on the terrace of the only bar in a one-bar town can produce sightings of combines, tractors, children who are not homeless running in packs, dogs of unknown parentage and merry disposition, and other sights uncommon in the large cities.
One that’s particularly famous is the terrace of Café Zurich on Barcelona’s Plaça Catalunya. Watching famous artists who happen to be in town watching their fans watching them is always good for a few giggles.
At a table of an outdoor bar in Bologna, Italy’s city centre on one of the pedestrianized streets. Locals, tourists, students and various others. People engaging in everything from a quiet stroll to an animated vocal discussion to dashes to catch their bus or train.
I don’t do much people watching these days as it seems I spend an inordinate amount of time either in my car or at work. One of my favorites used to be the lobby of a huge hotel at the waterfront in Maryland.
They have ample benches and chairs for quick meetings so you don’t stick out and since this hotel basically caters to business people, I can focus on watching adults going about there day.
The business part makes it more interesting for me as you get a greater variety and also they have different groups (conventions etc) who make watching interesting.
When we were living in San Jose there was a hotel there that was a perennial favorite for BayCon, a large regional con. Since we lived there, we never went to the hotel except for BayCon, until one year, the company Christmas party was there. Being heavily into the holidays, not only was the lobby much less crowded than we were used to, they were all stuffy business-types. We commented on how different it looked without the fen.
From the comfort of an Ikea restaurant with a plate of swedish(-ish) meatballs, looking down on the loading zone, and taking wagers on whether the laden trolley will fit into the compact car, with or without unpackaging.
Sturgis motorcycle rally. Sit anywhere in the vicinity for hours of jaw-dropping people watching. You can see some hilarious things. For example, sit on a wooden walkway in Keystone, SD and you’ll see the jarring disposition of a family of four, enjoying a family vacation, attempting to cover the eyes of their two young children as women walk by wearing thongs, leather chaps and electrical tape covering their nipples. Every summer.
I have the type of intent resting insane face that puts people off. Any time I’ve tried people watching, or just spacing out in a public place to enjoy a breeze and gentle sunshine, there’s always someone who wants to take me up on the fight I’m clearly instigating. Or maybe I just look like an easy target for some random stranger abuse. Now, I’m not a fast thinker and I learned a long time ago to avoid physical or verbal altercations because I always make a mess of myself. So my alternative is to not go out unless I’m on a mission, and to actually see as few people as possible.
Any outside bar on Walking Street in Pattaya, Thailand.