What is also fun is to just say 'hi' or 'hello' to random strangers, accompanied by a smile. I usually do that on the sidewalk (western Europe), but other places might also be fair game. I made an active effort to just greet people, now it's a habit. In the beginning I felt rejected sometimes by people blankly staring back at me and not saying 'hi' back.

Some people are not into that, don't suspect it or don't know how to handle that. Like half of this comment section, I really read some cringe comments here about how people don't want to be talked to. To them I can only say: don't worry, I see/feel from miles away that you don't want to be talked to. And even then, I might say 'hi' to you, just to gauge what happens. ;)

For me, it's okay if people don't say anything back. That's not even the point. I want to share a little positivity with the world (when I'm in the mood) and 90% of reactions are either a big smile back, a little nod or even a small conversation. There's nothing for me to 'win' (as a lot of commenters seem to think is the point); thinking in these terms makes it a business transaction.

I found out that especially elderly people are way more open to these things. Also, people who don't seem open to it (probably going through a bad time) might actually surprise you with a warm and welcome smile. These little interactions taught me to not judge a book by it's cover, give a little without expecting anything in return, and just knowing that you made somebody smile who needed that.

So if you think this article is something you want to try, but you're a little afraid thinking about whole conversations and stuff, just start with greeting random people. Don't see it as a transaction, just see it as age old human behavior (which it is). If you have a hard time reading social clues, this also might be a good way to practice that a bit.