The club scenes where Randy takes Julie were filmed in a Sunset Strip club originally named Filthy McNasty's in the 1960s and '70s. In the '80s it was called 'The Central,' which was later purchased by Johnny Depp and is now known as The Viper Room, where River Phoenix infamously died on Halloween, 1993.
Nicolas Cage had a crush on co-star Deborah Foreman. He later admitted in an interview," I just adored Deborah, so there was very little acting on my part." Cage would write her poetry on set, and one poem was titled, "American Girl," which Foreman still has to this day.
When Lee Purcell (Beth Brent, Suzi's stepmom) says "Plastics" to Skip, it's a reference to Il laureato (1967) about a younger man's affair with an older woman. She's obviously throwing him a hint that she's trying to seduce him, like in that movie (which he obviously isn't familiar with). Likewise, the end of Valley Girl (in which Randy crashes a formal event to which he was not invited and runs off with the prom queen) clearly references the end of The Graduate, in which Benjamin crashes Elaine's wedding and runs off with the bride. The closing shots of Randy and Julie in the backseat of the limo echo the closing shots of Ben and Elaine in the back of the bus, and Julie's white lace prom dress is even very bridal.
The producers of this film approached Frank Zappa about making a film based on his hit single "Valley Girl" (released May 1982), but he refused, leading the producers to make the film without his involvement. Zappa later sued them but lost the case.