During the scene where Leslie recites the poem to the children and Ben removes her from the room, they are dressed as Westley (as Dread Pirate Roberts) and Buttercup, and one of the children is dressed as a Rodent of Unusual Size - all from The Princess Bride (1987).
During the haunted house when Leslie (Amy Poehler ) is talking to a group of children she says "nothing gold can stay". This is a reference to a poem by Robert Frost made famous by S.E. Hinton's novel "The Outsiders". Rob Lowe (Chris Traeger) played Sodapop Curtis in The Outsiders (1983), the movie adaptation of the novel.
While Donna is showing the audience the products that are "trending" at Annabel Porter's event, you can see a Lush bath bomb "Think Pink" at one of the tables, which has since been reformulated.