Do they need to know specifically that I’m the one who did it? If so, I stand by my “Message in the stars” miracle for this. If not, I’m gonna go for a more useful miracle.
Jesus could not accomplish what you ask and was noted as saying
“He said to him, ‘If they do not listen to Moses and the Prophets, they will not be convinced even if someone rises from the dead.’”
And out of 10 men healed via a miracle of God, only one actually came back to thank Jesus and praise God.
12 As he was going into a village, ten men who had leprosy…14 When he saw them, he said, “Go, show yourselves to the priests.” And as they went, they were cleansed.
15 One of them, when he saw he was healed, came back, praising God in a loud voice. 16 He threw himself at Jesus’ feet and thanked him—and he was a Samaritan.
17 Jesus asked, “Were not all ten cleansed? Where are the other nine? 18 Has no one returned to give praise to God except this foreigner?” 19 Then he said to him, “Rise and go; your faith has made you well.”
We are trained not to see miracles, to convince ourselves it can not be true and accept something else. It takes a person of faith to actually see and to actually hear. It is not in the miracle itself or the working, but in the faith. My own personal opinion is it our free will that prevents us from seeing the glory of God.
I tell you miracles happen all the time even today, and is woven into out society and keeps things running, but the scriptures are true so that, “’they may be ever seeing but never perceiving, and ever hearing but never understanding; otherwise they might turn and be forgiven!’”
That is…a horrific idea, to be honest. Hell on Earth would shortly ensue.
I admit I’m still baffled why one would want to reveal oneself as a miracle worker to the world. But in the interest of not fighting the hypothetical:
Does giving other people powers count as “directly affecting the minds of other”? Because never mind Klingon, I’d give everyone in the world the ability to read, write, speak and understand every language in the world (including the various sign languages and those with no remaining living speakers - okay, and Klingon too), and make it hereditary. Sort of a reverse Babel thing, so that everyone can speak to everyone else.
It’d put a bunch of translators out of a job though. But on the bright side, you’d never have to press 1 for English again.
I believe the the OP’s third restriction:
Has to be carefully taken in context. Cure all the sick? That’s great even though it puts a lot of health care people out of work. Create a free source of clean energy? That also would affect a LOT of people.
Having said that, I’m in favor of dramatically curing the sick and restoring lost abilities. I believe that Broomstick’s suggestion has the most merit:
My ideas were more mundane, but on a larger scale, than those mentioned. Maybe creating a small area of a few hundred square miles of tropical rainforest directly over the South Pole, with the carbon coming from atmospheric CO2, enough to reduce things to where we were at in pre-industrial times. It might also be interesting to do something like transmogrifying the moon so that it’s actually made of cheese, with the appropriate change in its mass so that its orbit isn’t affected.
So world wide unblockable stellar pop-up ads every time you go star gazing? I thought he said nothing evil. 
What would you do with all the excess oxygen? Carbon in atmosphere is CO2.
I’ll take it a step farther - if I am required to spread the good word as a public miracle worker, I’d flat out refuse to perform miracles. I’ll not transforming MY life into a living hell of notoriety for the sake of doing good deeds. I’m just not that selfless.
As long as I get to retain my anonymity.
Even so I’m afraid that humans are simply too irrational. Any widely accepted (and therefore global or close to it) miracle is going to trigger a rash of crazies, one way or another. I don’t want to be responsible for starting religious wars and unleashing cult murders. I think I’m just way too cynical for this particular thought experiment.
But I do like this one.
By this standard, nothing could ever be done; nothing is harmless. No more threat of war = a lot of soldiers are now unemployed. No crime = cops out of business. No pollution = the pollutant-cleaning folks are out of business. At a certain point you just got to say “the benefits outweigh the drawbacks”.
There won’t be that much excess oxygen - the current proportion of CO2 in the atmosphere is just slightly above 0.04%, even if all of it was converted to O2, it would still be a rounding error compared to oxygen’s 21%.
Heck, at other times in Earth’s history, the O2 levels have reached as high as 35%!
Referring to restriction "C’ in the OP:
Thank you Velocity, that is just what I was trying to state.
“TENS OF THOUSANDS KILLED OR INJURED AND BILLIONS IN DAMAGE CAUSED BY HAIL OF GIANT DIAMONDS. MASS RIOTING ENSUES OVER OWNERSHIP. GLOBAL DIAMOND MARKET CRASHES DUE TO SUDDEN GLUT.”
Create a universal language that is implanted in everyone’s brains, BUT, to ensure that I am identified as the source, in the beginning people would only understand what I say - and I would tell them exactly when the switch would be “turned on” and everyone could talk to each other.
After that, for fun:
Decode the Voynich Manuscript (or provide indisputable proof if it IS nonsense).
Reassemble the lost reels of the movie “Greed” from wherever the atoms went when they were destroyed.
Bring back the passenger pigeon.
OK, so the miracle needs to be something that clearly violates our existing knowledge of what the universe is and how it works (so it can be unambiguously regarded as a miracle), and it needs to clearly identify the miracle worker, and it needs to reach as broad an audience as possible.
@Left_Hand_of_Dorkness is on the right track then with his “message in the stars”. Absent that, I would have suggested something similar with the moon, e.g. make it several times larger than its known size and make it a full-color video of me holding up a sign saying I’m the one who did this.
I another way to go would be to have the message “Call Oredigger77 at 555-1234” appear on every surface the G7 leaders look at for a day would work too. Once they call I just tell them the message from god that I want to deliver.
Having all of them do something simultaneously that was good for the world and giving me/god credit should move most people’s belief.
One miracle? OK - everyone in the world wakes up on the same date with a tattoo that spells out in the local language “Hi, this is Miracle Worker with a message for you - Have a nice day.”
This makes it personal - someone with any pretention to sanity can’t claim that “It’s a fake because I didn’t see it.” And in the aggregate so impossible that it has to be recognized as a phenomenon outside of the regular course of nature.
Or maybe the message should be “Hi, this is from I once was a Miracle Worker - I’ve shot my wad with this message.”