Harold and Maude from 1971 (IMDb, Wikipedia)
Directed by Hal Ashby (IMDb, Wikipedia)
Almost the entire soundtrack for the movie is by Cat Stevens.
"They met at the funeral of a perfect stranger. From then on, things got perfectly stranger and stranger." - movie tagline
PS. The following stills are only from the beginning/middle of the movie. I wouldn't want to spoil the joy for anyone who hasn't seen it. I only want to tease them into watching it.