When everyone is smoking on the roof, there's a Tim Horton's coffee tin on the table being used for the butts. A nod to Robin's Canadian-ness.
The reaction shot of Ted's future kids after the revelation of the gang smoking is the same as the one shot for the pilot episode. All subsequent shots show them in different clothes with an altered set. It's likely that this was the only one they could find to match the situation as all reaction shots for the kids were finished shooting early on in season 2 and they didn't have one that matched.
Ted's kids act shocked that the gang all smoke but in season 2, at Lily and Marshall's wedding, Robin and Lily go outside to smoke.
Another banner for the band "Pitter Pals Revival" can be seen behind Robin when they are all outside.