What is the significance of the Coke ad?

Unless Matthew Weiner decides to elaborate on it, we can only speculate, like the reviewer at The AV Club:

The final scene shows Don chanting in harmony with his fellow meditators. “The new day brings new hope,” says the group leader. “The lives we’ve led, the lives we’ve yet to lead. A new day. New ideas. A new you.” The image of a newly calm Don gives way to the famous “Buy The World A Coke” ad. The singers wear the loose hippie garb favored by the visitors at the self-improvement retreat; a couple of them bear a resemblance to specific people who Don encountered. A woman with ribbons in her braids, for instance…

…looks a lot like the woman at the front desk who tells Don that “people are free to come and go as they please”.

The implication is that Don went back to the only home that would have him — McCann, where Peggy extended a perpetual welcome — and applied his newfound insights to launch an iconic Coke campaign. The ad came about once Don understood that he wasn’t alone in his loneliness. After a decade where earnest ideas of free love and deeper understanding were juxtaposed against assassinations and spectacular societal discord—dreams that carried their own disappointment in tow—Don’s pain was simply a pronounced and individual iteration of a hurt that pervaded the culture. So Don promises to assuage that anxiety, with soda pop.

Jon Hamm weighs in:

In an interview with The New York Times on Monday, Hamm says there “probably” is a correct reading of the final scene, though he notes that it was a “a bit ambiguous.” Viewers of the acclaimed AMC drama saw Don meditating at a retreat in Big Sur, uttering a serene “om,” and issuing a small-yet-revelatory smile before Coke’s famous 1971 “I’d Like to Buy the World A Coke” commercial closed out the series. He says that he “was struck by the poetry of it,” though when series creator Matthew Weiner first shared with him the concept for the closing image, it did not come with context. “I didn’t know how his plans were, to get Don to this meditative, contemplative place,” he said. “I just knew that he had this final image in mind.”

[...]

My take is that, the next day, he wakes up in this beautiful place, and has this serene moment of understanding, and realizes who he is. And who he is, is an advertising man. And so, this thing comes to him. There’s a way to see it in a completely cynical way, and say, ‘Wow, that’s awful.’ But I think that for Don, it represents some kind of understanding and comfort in this incredibly unquiet, uncomfortable life that he has led.

Update: Matthew Weiner confirmed the theory in a discussion with novelist A.M. Homes on Wednesday at the New York Public Library:

Yes, Don Draper created the Coke ad. The last scenes of the series features Don hugging a stranger at a retreat and meditating with hippies, before the episode cuts to the 1971 Coca-Cola "Hilltop" commercial, to infer that Don returns to McCann-Erickson and creates that ad. "I have never been clear, and I have always been able to live with ambiguities," said Weiner. "In the abstract, I did think, why not end this show with the greatest commercial ever made? In terms of what it means to people and everything, I am not ambiguity for ambiguity's sake. But it was nice to have your cake and eat it too, in terms of what is advertising, who is Don and what is that thing?"