tldr: some free ways to benefit others
Epistemic status: Some things I noticed, made up or let ChatGPT generate

Idea

What if you had a button that you could press to make other people happy? Pushing the button doesn’t cost you anything besides the negligible effort. How often would you press it? If your answer is somewhere in the range between “Very often” and “Obviously I would build a robot arm that could press the button as fast as physically possible”, then there probably are a couple of things you should be doing more often:

Examples

• Give genuine compliments. I made a trigger-action plan that if I like something about someone and it’s not inappropriate to say, then I say it. Note: The more unusual the compliment, the better your judgment about appropriateness needs to be. 
• Share relevant knowledge and resources. Trigger-action plan: If I encounter something that could be useful, then I quickly think about who could also benefit from this and share the info with them. You can also do this with things you already know.
• Say thank you. Trigger-action plan: If I found something beneficial, then I say “thank you”. Don’t limit this to personal favors! A “thank you” in response to a talk, a public Slack message or a nice looking garden is very welcome, especially because it rewards providing common goods. When many people benefit from something and could in theory say thank you, often nobody ends up doing it. Saying “Thank you” to people who “just do their job” is neglected as well. Also, you can thank people for who they are (compared to what they did). You have to know and like a person rather well to be able to do that, but it’s super wholesome.
• Introduce people to each other, if they consent. Trigger-action plan: If I notice complementary interests in 2 people, then I first ask both of them (explicitly saying they’re allowed to say “no”) and if they say “yes”, then I connect them. You could connect applicants and employers, students who want to learn the same thing, or travelers who want to go to the same place.
• Let people know you thought of them. Trigger-action plan: If something made me think of somebody (positively), then I let them know. Very easy to do, very wholesome.

Caveats

• It’s possible to overdo some of these things. But you can probably always ask if it’s too much
• Maybe some of these do have a non-trivial cost in the form of mental effort. I don’t think they do for me but maybe it’s different for other people.
• Someone pointed out that there might also be a cost to your social status if you do a lot of the “Be exceptionally nice”-things like giving a lot of compliments, especially if you don’t receive the same niceness in return. While I can’t rule it out, I can’t confirm it either.

Conclusion

That being said, I still think these things are worth doing and are currently neglected, so I want you to do them more often! And I want you to comment all the happiness buttons I forgot to include in this post so we can have a nice collection, a common good of common goods.
Please include “#happinessbutton” in every comment that adds more happiness buttons, so they are easier to find.