The wood chipper seen midway through the movie belongs to 'Biff's Tree Removal' service. This is a reference to the character Biff Tannen from the 'Back to the Future' (1985) trilogy, and his famous line "Why don't you make like a tree and get out of here!"
Tree's freefall montage of various suicides, landing her back into Carter & Ryan's dorm room pays visual reference to the music video of the song that plays over the montage: Paramore's Hard Times where in the music video lead vocalist Hayley Williams, sporting similar attire to Tree Gelbman and blonde hair collapses slowly back into a bed from the roof.
Like in the first film where Carter announces the story bearing similarities to the plot of Groundhog Day, he establishes the similarities with this sequels' plot to Back to the Future Part II (1989). There are also some small nods to the film, like the wood chipper being from "Biff's Tree Removal", and also a clock tower scene.
The device Ryan has invented is called "Sissy," very likely a reference to the character Sisyphus in Greek mythology. Like Tree, Sisyphus escaped Death's clutches, and as his punishment in the afterlife, he was forced to roll a boulder up a hill every day for eternity, only to have it roll away from him back down the hill before he could get to the top. The story of Sisyphus is superficially like that of Tree who must relive the same day again and again only to see something finally go wrong each time before she can get out of the time loop (until obviously, unlike Sisyphus, she manages to finish her task and get freed).
Peter Jaymes Jr.'s officer victim character in the film, is the only newly introduced character in the film who has a death in the film, as newly introduced characters Dean Bronson, Dre Morgan and Samar Ghosh do not die at all in the film.
The trivia items below may give away important plot points.
The reveal at the end of Gregory Butler as the baby face killer, in the alternate reality, is a nod to one of the original endings of the first film where he had teamed up with Lori to kill Tree. Also having Stephanie Butler involved with the killing is a reference to the first film's alternate ending that was removed after negative test screenings.
Jessica Rothe said her favorite death in the movie was falling out of the plane, in a bikini, without a parachute and landing in front of Carter and Danielle. This death was also not part of the original film and was added during re-shoots.
Minus the addition of Stephanie Butler, Ryan Phan and the new secondary officer guarding John Tombs, all the slasher film body count of the sequel comprises of the previous films' characters that perished in various time loops: Tree, Carter, Lori, John Tombs, Tombs guarding officer and Dr. Gregory Butler.
The first film only featured Tree Gelbman die more than once, in the film's multiple time loops, all other victim characters, including the Babyface killers Lori Spengler and John Tombs' deaths were all once in different loops, Lori's being the only death not reset. In this film not only does Tree have multiple deaths, but as does Lori Spengler (who in the alternate timeline is merely a victim), Babyface killer John Tombs and newly introduced, Tombs guarding officer victim, who dies twice.