The Darth Vader scene, where Marty sneaks into his fathers room at night, was much longer, originally.
I read the Part II script. first draft, I believe, and it is almost completely different. It begins the somewhat same, minor stuff, a few scenes not on the movie, it is recognisable. Then, instead of going to 1955, they go to 1967, where Marty must help his mother earn money to go to San Francisco so he can be concieved, because she spend her money getting him out of jail, wanting to do someting nice for what she thinks is a draftdodger, since he has no papers, while he must try to prevent 1967 Doc from meeting 1985 Doc. There is really nothing from 1967 that survives in the finished movie, though the grand finale, driving the Delorean over a cliff, was kinda sorta reused in the third movie. In the end, Doc tires of the fourth dimension, setting his sight on the fifth.