What are some fun pranks for a sleepover?

Buy a few realistic looking rubber snakes and spiders to hide among the covers.

A little fake pile of plastic/rubber dog crap. Some are surprisingly realistic looking and moreso if you wet them down a little before placing.

Depending upon whether your toilet facilities are in the camper or in the home placing a clear sheet of cling wrap over the bowl (but underneath the seat) can be fun. Just make sure there aren’t any creases in the surface to give it away.

Have fun.

My advice be careful with the cling wrap one. I’ve never had it done, but depending on who you do it to, the person sitting in a pile of their pee may not be too amused. You may also have pee on the floor, and who’s going to clean that up?

This is 2016. Change the WiFi password.

Get a Dinosaur costume and do this

I don’t care for pranks. By definition they split friends into two groups: the ones in the know and the outsiders. From that day forward the one who is pranked, (the prankee) begins planning a revenge prank on the one who planned the original prank (the jerk prankster) and the cycle continues.
It is far better to have a circle of friends you can trust.