It's pretty obvious that this series is based on one of the most popular authors in the world. From the group of pre-teens in It to the supernatural girls of Carrie and Firestarter, King's fingerprints are all over this series. There's even a scene where one character is reading Cujo and another character says how great it is. Definitely check out the audio books available from Overdrive if you don't have time to sit and read.
While Stranger Things might lean into the horror, it definitely owes some of it's wonder to Spielberg. While most seem to lean toward the E.T. elements of the series, there's definitely something to be owed to Close Encounters of the Third Kind. A horrible wonder about the aliens, the other side, and one character slowly drowning in their own obsession definitely bring to bear the otherworldly atmosphere.
Another feather in the hat from Spielberg (this time as producer) and also mentioned in the series, Poltergeist features the story of a haunted house that soon turns terrifying as a little girl goes missing. The supernatural threat as well as dark portals to the unknown feature heavily as well as the teen and family dynamics. Plus, there's an evil tree in both.
Because no story about preteens would be complete without talking about Goonies, here they are. While the story and characters might not hold up as well as others on this list, it's still a fun adventure tale for the entire family. The gung-ho, save our friend's life attitude of the kids in the story is a direct coorelation to the "let's get some pirate treasure to save the town" attitude of Goonies. I would like to say that if Mikey, Data, Mouth, and Chunk had found a girl who could move things with her mind they would have acted the same way.