Full Moon Fever borrows most of its plot from Chicken Chicken — the protagonists, one boy and one girl, tick off an old lady and subsequently find themselves transformed into animals.
Taken to ridiculous lengths with the number of summer camp stories:
Welcome to Camp Nightmare (Original Series #9): A boy goes to a summer camp said to be haunted by a monster in the woods and his bunkmates keep getting injured and disappearing (Turns out the place is a Secret Test of Character for the protagonist and that he and his family are human-looking aliens ready to go on a vacation to Earth).
The Horror at Camp Jellyjam (Original Series #33): A brother and sister on a road trip crash their trailer into a sports camp where everyone is obsessed with competing and winning in sports so they can be slaves to a giant purple blob whose cronies are the Brainwashed and CrazyStepford Smiler counselors.
Ghost Camp (Original Series #45): Two brothers go to a summer camp where everyone is a ghost and the only way they can escape is to possess the body of a living being.
The Curse of Camp Cold Lake (Original Series #56): A girl at a water sports camp finds herself haunted by a murderous Yandere ghost girl who can't go to the afterlife unless she has a buddy.
Fright Camp (Series 2000 #8): A group of horror movie fans win a trip to a camp that is said to be home to a horror movie director's creations.
Return to Ghost Camp (Series 2000 #19): A boy switches places with another camper on his way to the ghost camp from the original series and enjoys living his life — until it's revealed that the boy the protagonist switched places with is prepped to be sacrificed to a monster in the woods.
Escape From Camp Run-For-Your-Life (Give Yourself Goosebumps #19): You (the reader) find yourself in a camp filled with zombie children.
Welcome to Camp Slither (HorrorLand #9): A brother and sister find themselves in a camp infested with snakes.
Let's Get Invisible!, Mirror Mirror On The Wall, and The Ghost In The Mirror all involve beings that try to switch places with people by using mirrors. Mirror Mirror On The Wall is basically just Let's Get Invisible! without the invisibility angle added in.
Horrors Of The Black Ring is more or less The Haunted Mask without the symbol of love angle, since both are about cursed objects that cause the wearer to gradually turn twisted and evil.
Goosebumps Wanted: The Haunted Mask reuses The Haunted Mask's plot though with a few different details here and there.
Goosebumps Gold was a series planned after 2000, with three titles, The Haunted Mask Lives!, Happy Holidays From Dead House, and Slappy New Year, established. Cover artwork for the first two can be found on Tim Jacobus's website, and certain websites stock some of the books for sale. The plot for The Haunted Mask Lives! would've been about Carly Beth being targeted by the novelty shop owner who initially made the masks. Slappy New Year was included in the Horrorland series.
Goosebumps 2000 was supposed to have a book called "The Incredible Shrinking Fifth Grader," but the series ended before it could be published. "The Incredible Shrinking Fifth Grader" was also supposed to be part of the aborted Goosebumps Gold series, but eventually found its way to the Goosebumps Horrorland series under the name "Night of the Giant Everything."