The Other Darrin: 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. Dana Barron is the only one to return to the role; she plays Audrey in the Cousin Eddie Christmas sequel.
Today, finding out if the park is closed would be as simple as checking its web page, and because of the prevalence of GPS, getting lost driving across the country would be much more difficult.
Also, Clark's payment problems after he's strongarmed out of his cash would not be that much of a problem nowadays considering that debit payments from his bank account is common and credit cards are accepted in far more places now.
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.
The Griswold's 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.
The ending was originally completely different. Clark would hold up Roy Walley in his home and force him to sing and dance, and the police arrive to arrest Clark, when it's revealed that the mysterious woman in the red car is actually Roy's daughter. She convinces her father to drop the charges and the family goes on a flight back home after receiving Walley World hats, but they discover that they caught the wrong flight and Clark forces the pilot to turn the plane around. Negative test screenings made the filmmakers rewrite the scene.