One of the original ideas for the film in The Eighties was a musical remake of the Disney animated version of the original play starring Michael Jackson. Supposedly, Jackson never forgave Spielberg for not casting him in this version.
Kevin Kline was originally cast as Peter, but Soapdish reshoots scuppered this.
Steven Spielberg was not the first choice to direct the film. Nick Castle had planned to direct and spent a year in pre-production writing the script and planning. Spielberg then became interested and Castle was paid to leave the director's chair. He is still credited for the screenplay though.
This was intended to be a musical, but according to lyricist Leslie Bricusse, the filmmakers realized it would be both too long and too expensive, so most of the songs were cut ("We Don't Wanna Grow Up" and "When You're Alone" the exceptions). The happy instrumental theme that closes the film was originally the melody of a song called "Childhood", and a sheet music anthology of his work includes it, with the explanation that everyone loved it but couldn't think of a way to keep it in the film.