I don't live in a densely-populated enough area to really be able to even "see someone on the street". 20 minutes to civilization from my house.

If I end up going into a store or some shit at the same time as someone, I'll give them a quick pleasantry or something.

Although if I see someone driving down my country road while I'm on it, I'll generally wave at them through the windshield. It's only polite.

I say Hi to everyone in my Street, any Servicemen and other people that I see regularly, as well as people like the bus driver (same guys every time).
Most Start smiling when they see me so even when I had a shit day that makes it a little better.
Also some guys who apparently live in our Street (only know them from being them) recently talked to me, they were nice people and they said it was nice that I greeted everyone.

I live in a village though so it kind of is easier. In a City I would feel autistic to do that to other people than my neighbours

just smile at people as you walk past