As HBR said, a lot of the free raoks are things we should be doing anyway. My daily one is to always make sure that the new people in work aren't left out of anything—always go and say hello to them in the morning, make sure they're invited to lunch, etc.
One that I've seen done, but never had the
cojones to do myself is, "Hi, I just
had to come over and tell you that you're super cute. [big smile] Well, have a nice day." Note that this ceases to be a raok if you go on to get the person's number.
A less terrifying prospect than the previous one, but similar: tell a stranger you like their shoes.
Whenever I stop to watch a street performer I follow two simple rules: 1) It's not free, it's just the honour system. If you stop to watch, you pay. 2) If it's worth some of your money, it's certainly worth taking a second to tell the person that you enjoyed their performance and to shake their hand.
Expanding on thanking people who are "just doing their job", I think it's important to make a particular effort to interact with anyone in a service role in a proper human way. It's unfortunately very easy to treat a person like a robot, especially shopkeepers and bus drivers. Some people even manage to work in the "thank you" in a robotic way. Don't do that. Say hello, chat to them, look the in the eyes and smile when saying thank you instead of mumbling it as you fiddle with your change.