Alice May's physical appearance seems based on Gwen Stacy, the first girlfriend of Peter Parker/Spider-Man (Blonde hair, black headband, green striped shirt).
The final time where Casey Kasem, Nicole Jaffe and Heather North voice together as a group, the characters, "Shaggy", "Velma" and "Daphne".
The monster that they mention known as "The Creeper" originally appeared back in 1969 in The "Scooby Doo Where are You" episode "Jeepers its the Creeper".
While Fred and Daphne are dancing at the prom, two background dancers look like Coil Man (blonde Beatles style haircut, tubby) and Multi Man (tall, moptop red hair that covers his eyes) from Frankenstein Jr and the Impossibles. The Coil Man look-a-like is in a shot of Daphne and Fred, while the Multi one gets separated from his date when Scooby comes running in.