At the post office boxes, Joe comments to Peter that he receives his Seinfeld (1989) residuals there. In 10 episodes of "Seinfeld" (1995-98), Patrick Warburton played Elaine's boyfriend, David Puddy.
In the last scene there is a flashback to Peter and Lois's honeymoon. They both have their Rachel and Chandler look as first seen in Peter & Lois' Wedding (2019) earlier this season, which itself is a flashback paying homage to Friends (1994) and other 90's nostalgia.
In a shot that pans down from the street to where the guys are stuck underground you see the same skull (Chief Diamond Phillips) that Peter discovered and later reburied in Season 4's Petergeist (2006).