When Fred mentions putting a coin into a telescope, Velma comments, "They're francs." Fred then says, "I thought they were mine". This is a reference to Fred's voice actor, Frank Welker.
"Ready to scare" is a spoof of the common fashion industry term "ready to wear." In this case, it's probably a specific spoof of the movie Ready to Wear (1994), which revolves around the hi-jinks and mishaps surrounding a Paris fashion extravaganza.
Sonny Les Matines' name sounds like a line in the 18th-century French children's song "Frere Jacques", about the ringing of church bells. Scooby and Shaggy recite a line from this song when he tells them his name.