All the costumes seen at the Criminology Museum were fully functional, meaning they could all indeed be worn. The plaques accompanying them also included trivia on the specific costume based on the original cartoon.
When Velma is sitting in the backseat of the van and her leather outfit makes a farting noise, the line "That was my outfit, I swear," was an ad-lib by Linda Cardellini.
A third installment was going to be released, but due to not being as financially successful as the first film it was scrapped. According to James Gunn, the mystery gang were going to Scotland to investigate a series of monster attacks, only to discover that the monsters were victims and were being controlled by an evil human, which would have forced Scooby and Shaggy to confront their own prejudices about monsters.
Velma's nameplate necklace reads 'Velmster.' This is the nickname Fred calls her in Scooby-Doo (2002).
At one point in the film, Scooby and Shaggy are pretending to sing into a toilet brush. The song they are singing is "Strangers in the Night" - Frank Sinatra's version featured the improvised scat lyrics, "scoo-bee-doo-bee-doo," lyrics which then-CBS executive Fred Silverman chose as the name of the new cartoon series Scooby Doo, Where Are You! (1969). The original name for the dog was "Too Much," a popular catchphrase of the era.
Neil Fanning: At the Faux Ghost, Scooby spits an egg into the cup of the thug next to him. The thug is played by Scooby-Doo's voice actor for this movie.