It was piled obscenely high, and fizzing down the far side of the glass. Clearly designed for those who had always secretly thought that coke wasn't sweet enough, it added layers of yellow ice-cream, soft gloops of cream and a demure swirl of pink-flavoured sauce on top. There was a pint of it. I'm not sure why they didn't go the whole hog and make it Irish - it seemed uncharacteristically restrained for the guilty hands behind the diner's counter not just to add a slug of Jameson's and be done with it.
We were sitting outside in the sunshine, and I'd rolled down my knee-length socks to tan my magnolia legs. I was on tour guide duty, and as usual I'd managed to programme exactly the sort of day I wanted to have. We'd taken only semi-ironic photos in Times Square, then wandered up Broadway to Columbus Circle. Being inside the park made me breathe easier, although as always with my guide hat on, I'd found myself scanning around for possible insufficiencies. Aren't those daffodils a little small? Weren't there usually more squirrels? Is that hot dog going to give an authentic New York hot dog experience when we can't actually see the onions frying on the hot slab?
But it was the Conservatory Water that really made me catch my breath, from both sides of my Janus-face (one half looking out appreciatively as an evangelical new New Yorker, the other darting around to check she hadn't over-sold this once-in-a-lifetime experience). There were toy boats on the pond. Dozens of them. For a moment I was taken back in time before cheap long-haul flights and digital cameras; before ice-cream floats even. I saw Central Park as it must have been when it first emerged from the wasteland above 60th street, landscaped and templed to within an inch of its fresh, fake new life. I saw a boy dressed in a sailor suit, putting down his stick and hoop to watch a miniature sail boat cross a tiny lake.
Then I saw the stall. These boats were not owned by eccentric toy-spotters or bequeathed by generous uncles - they were rented by the hour. For a second the sun went in behind a cloud, and the boy with the hoop disappeared.
Later in the cafe, I thought how much he would have enjoyed a sip of my coke float. I would have had plenty to spare, after all. But now we were on Amsterdam and a Duane Reed on the corner was intruding on my Upper Westside story, so it was time to pull up my bobby-sox and continue the tour.