Often it's the camp counselor. They seem normal at first but then turn out to be a psycho. They have easy access to the campers and can use their position of authority to carry out their evil deeds.
Such a character could be very adventurous, always seeking new experiences at the camp. Or, they could be quiet and introspective, observing everything around them. Maybe they have a fear or phobia that gets challenged during their time at the camp.
Common themes include the breakdown of trust. Everyone at the camp starts suspecting each other, which adds to the horror. There can be themes related to the past coming back to haunt. For example, an event that happened years ago at the camp resurfaces in a terrifying way. And then there's the theme of being cut off from the outside world. No cell phone reception, no easy way to get help, making the situation even more dire.
One popular theme is friendship among the campers. In many fanfictions, characters form strong bonds during their time at the sleepaway camp. Another theme could be mystery, like trying to solve strange happenings around the camp at night. Also, romance often plays a part, with campers developing crushes on each other.
The outcast camper is always an interesting character to focus on. This could be a camper who is shy, different from the others, or has some sort of special ability or disability. Their journey of trying to fit in or stand out at the sleepaway camp can be a great plot for a fanfiction. You can show how they change and grow throughout their time at the camp.
Once at girls camp, we had a talent show. One girl tried to do a magic trick but ended up making the whole tent full of feathers from a pillow she accidentally ripped. It was hilarious as we all had feathers in our hair and clothes.
One time at band camp, our conductor tried to demonstrate a really energetic movement but ended up tripping over a music stand. It was hilarious as he just laughed it off and continued with the lesson. Another story was when a newbie in the band accidentally played a completely different tune during a practice session, which threw everyone off but also made us all burst into laughter.
One story is about a recruit who accidentally put his boots on the wrong feet during a morning drill. He was running all lopsided and the drill sergeant couldn't help but laugh while correcting him. It was a really funny sight as he tried to march in formation with his boots in the wrong place.
At a particular camp, the counselors planned a treasure hunt. One team got so lost that they ended up in the staff quarters by mistake. They were looking everywhere for the 'treasure' and were really confused when they found themselves among the counselors' beds. Their bewildered faces were priceless and it was a really funny moment that we still talk about at camp reunions.