European Vacation is filled with this for European audiences, as the supporting cast was drawn from comic and character actors from each country featured. Just to name a few: British comic actors Mel Smith (the London hotelier), Robbie Coltrane, Maureen Lipman (other guests in the hotel), Ballard Berkeley (the second motorist), and Eric Idle (the injury-prone cyclist), French character performers Didier Pain (the camera thief) and Victor Lanoux (one of Ellen's kidnappers), and German comedy veterans Willy Millowitsch and Erika Wackernagel (the Griswolds' unwitting German hosts).
Meanwhile, for American audiences, Pig in a Poke is presented by John Astin and announced by Gary Owens.
The Other Rusty & Audrey: While Chevy Chase and Beverly D'Angelo kept reprising their roles as the Griswold parents for the first four films, their kids were played by different actors each time: Anthony Michael Hall, Jason Lively, Johnny Galecki and Ethan Embry played Rusty while Dana Barron, Dana Hill, Juliette Lewis and Marisol Nichols played Audrey.
Throw It In: In European Vacation, as they're checking into a hotel, Clark accidentally refers to both Rusty and Audrey as his "daughters". This was a mistake on Chevy Chase's part, but was left in because it was so Clark Griswald-ish. If you watch closely, Chase has an Oh Crap expression afterward, but then shrugs it off and continues the scene.
What Could Have Been: The destination was originally supposed to be Disneyland, as it was in the short story, but it was rejected since the park is open year-round and thus the ending with John Candy wouldn't have worked.