(1) When I saw Don's smile my first words were "Don is back"! The Coke ad wasn't ambiguous, relatively speaking. It was as heavy-handed as Weiner was going to be for the hard-headed. Peggy offered Don Door No. 1 when she told Don to come back and then told us Don could go back if he wanted. We know Don likely took the offer by not taking Door No. 2 (suicide) or Door No. 3 (becoming a family man after swooping in to take control of the kids' lives). But, just in case you needed more---we are given a hippy dippy Coke ad set in the same type of hippy dippy setting where Don's satisfied smirk says goodbye for the show.
The ending reminded me immediately of an interview Weiner gave 2 weeks ago on NPR when the host speculated on possible endings in light of Don walking out of the McCan meeting to drive. At the time, there was ambiguity as to whether Don had quit the firm, and Wiener opined that Don was looking for his place in the new world and the still changing American culture. He suggested the way to look at what was going was, to paraphrase, "there is no doubt that Don has the capabilities to adapt to his and America's new world better than anyone, if he chooses to continue in advertisement or business generally, the question was only whether he wanted to and would."
That little smirk and the Coke ad was the answer. Don's character doesn't become a hipster because he cries it out with Mr. Cellophane in a commune. That man smirks for only two reasons---he knows he is going to get in a difficult conquests' pants or he just had a brilliant idea.
(2) Peggy certainly didn't make the Coke ad. The ending-point on her story arc signals that, while she was Don's protege the whole show, in the end she didn't make Don's choice. She's been struggling with life, just as Don has, by placing family entirely behind ambition and is, therefore, alone throughout the series (we know it's a real sacrifice for her by her bitter talk about giving up her child earlier this season). She gave up her baby at the hospital where Don visited her to exort her to return to work in the first season, but at the end of the series her choice is to sacrifice a little bit on the ambitious ego side for once and she gains on the family side soon after as a result.
(3) Don has nothing without his work. I'm surprised people would think he made any other choice other than suicide or advertising narcotics. Don contemplates quitting as early as season one, and he periodically runs away from it because, well, "Don does this every now and then." But he's never been able to stay away. Even when he was compelled to quit by his partners---sent off for rehab---he found ways to do the work by laundering it through Drunk Freddy.