Cheryl's comment #JusticeForEthel is a reference to the #JusticeForBarb trend related to Stranger Things' Barb, who is portrayed by the same actress Shannon Purser.
Greendale is mentioned by Cheryl. In the comics, this is the home of Sabrina Spellman.
As an Easter Egg, several characters that have made minor appearances in Archie Comics before are seen commenting on Chuck's Instagram post, Tony (Toni) Topaz, Frankie Valdez, Angel Angelino, Debbie Dalton, Marie Rodriguez and Cricket O'Dell.
Both Betty and Veronica mention going Full Dark, No Stars. "Full Dark, No Stars", published in November 2010, is a collection of four novellas by American author Stephen King, all dealing with the theme of retribution.
During the "Taste of Riverdale" event, Mayor McCoy quotes about the 75th anniversary of Riverdale. Coincidentally, Archie was introduced in 1941 in Pep Comics, 75 years before "Riverdale" episodes were shot (1941-2016).