The Red Stapler: Demand for Amber, and all dinosaur related toys, books and products.
Refitted For Sequel: Sort of. When they made video games based off the movies, and needed to stretch them out, they mined each film's respective book; the first SNES game is in many ways more based on the book than the film.
Then there's the fact that a couple of scenes from the second film (the vacationing family at the beginning and the T. rex sticking its tongue through the waterfall) were originally from the first book.
Stan Winston: The genius responsible for the live-action dinosaurs when the computer guys are taking a break.
Torch the Franchise and Run: A bit of a preemptive example, but the original Jurassic Park novel was never meant to have a sequel, so Crichton had Isla Nublar firebombed, killing all of the dinosaurs on it. That didn't stop Spielberg from convincing him to write a sequel, anyways.
Richard Donner, Tim Burton and Joe Dante were all considered to direct when studios had a bidding war for the material (Donner would have made the film for Columbia Pictures, Burton at Warner Bros. and Dante at 20th Century Fox). James Cameron also stated the rights were bought hours before his bid. In the end, Universal and Spielberg won out since Spielberg was Crichton's first choice to direct (and the studio used the upcoming Schindler's List, which Spielberg had been lobbying to direct, as incentive). But imagine how dark Burton's and Dante's versions would have been. (Cameron stated his version would be much more violent, but agreed that wouldn't exactly be better as "Dinosaurs are for 8-year-olds. We can all enjoy it, too, but kids get dinosaurs and they should not have been excluded for that").
William Hurt, Harrison Ford, and Richard Dreyfuss were all offered the part of Alan Grant. Julie Binochet and Robin Wright were both considered for the part of Ellie Satler. Sean Connery was offered the role of John Hammond. And Brian Cox auditioned for the part of Robert Muldoon.
Spielberg changed the endings of the first two films in the middle of filming. On the first, the film was supposed to end with the dinosaur fossils in the visitors' center falling on the raptors and crushing them. Would've been a tad anti-climatic. Spielberg realised that the audiences would never forgive him if he didn't bring back the T. rex for one last heroic moment, and so he did. With The Lost World, it was supposed to end with a Pteranadon assault on a helicopter. Spielberg had proposed putting a T. rex in San Diego early on, but was more or less ignored, until he insisted that it was the ending that would be filmed. It is quite safe to say that both endings that found themselves in the movie are better than the planned ones.
Universal's animation division created a pitchfilm for a possible animated series around 1995; it used a lot of then-state of the art CGI; the series never got off the ground. You can see it here.
Going off that, the Jurassic Park Chaos Effect toyline was original supposed to tie in with an animated series, but it never got off the ground. Exactly why it was canned is a matter of debate. Some suggest Spielberg/Crichton did not like the concept when it was pitched and canned it. Another report suggests that Speilberg was in a bad mood the day it was to be pitched to him and canned it before even learning what it was about.
Word of God: Malcolm was supposed to be dead at the end of the first novel. Since he survived in the movie and became a popular character, Crichton decided to include him in the second novel. The Never Saw The Body aspect merely let him get away with it.
Awesome, Dear Boy: According to IMDB, Samuel L. Jackson and Wayne Knight signed on to the film because they liked the idea of playing characters who get eaten alive by dinosaurs. Unfortunately though, both actors were disappointed that their characters' deaths were not shown in the finished film. Knight's character Nedry's death doesn't happen directly on-camera, and the scene where Jackson's character Arnold gets killed was cut out of the film entirely.
Creator In-Joke: "You're out of a job", "Don't you mean extinct?" was originally an exchange between Spielberg and Phil Tippett, after seeing an ILM cinematic proving that Tippet's go-motiondinos wouldn't be necessary (Tippett was still kept as an advisor).
Parental Fear: Lex and Tim's actors were picked because while watching hundreds of audition tapes, after dozing off Spielberg's wife jumped up when she heard Lex's actress scream, and thought one of her own children was in danger. Her husband was convinced they'd make an impact on the audience.