I just read your posting, so as of yet, I am without suggestions, but I think that is a wonderful way to spend Halloween. When you get back, you will have to let us know how it turned out. Obliviously, with camping, you should be able to incorporate the camp fire to help set the mood for many activities. Good luck!!!
Go to a Blair Witch type woods, or other deserted area, or camp in a graveyard if it's not trespassing. Very spooky. The dark and environment should lend itself to scaring everybody and all you need to do is cook over a fire, and tell ghost stories.
I did this MANY years ago... luckily, not campiing -- had an ancient farmhouse -- but there was an outhouse (so everyone had to go outside in the dark at least once) and woods, etc. We went for a walk, and everyone thought my cohost was delayed, but she was actually all made up in zombie makeup, waiting for a classing "boo!" (I think she got scared, out there in the dark all by herself). We also rigged a couple of simple pranks with bells and fishing wire, etc. The beauty part is that the outdoors & your guests' imaginations will do most of the work for you!
If you can come up with a plausible local legend, that would be cool. Our farm actually came with a real local legend, which I was able to share -- even if it was something that (supposedly) happened every 7 years, and it was the wrong year, I was able to tell about being up there, w/ my brother, and w/o our parents for an evening when we heard the story on the radio. It put everyone on edge.
". . . he would have passed a pleasant life of it, in despite of the Devil and all his works, if his path had not been crossed by a being that causes more perplexity to mortal man than ghosts, goblins, and the whole race of witches put together, and that was--a woman." Washington Irving, The Legend of Sleepy Hollow
We just got back from camping this weekend. Of course our camper was decorated with bat/skull lights & our jack o lanterns. Ghost Stories around the campfire are good as well as taking a night time walk in the woods with just your flashlight. (always creepy) I wish we could be camping over Halloween, but then there would be no yard haunt.
"Beware the Jabberwock, my son! The jaws that bite, the claws that catch! Beware the Jubjub bird, and shun the frumious Bandersnatch!"
Have a shadow contest. When my kids get into the tent to sleep when we are camping, my husband and I always take turns standing between the tent and the campfire and using our hands, sticks, leaves, etc. to see which of us can make the scariest shadow on the outside of the tent. The kids can see them from inside, and they either roar with laughter or get really creeped out.
When my sister turned 50 and I 40, at our party we were each given a basket. It contained string, pieces of thin wire, tooth picks, clay,(all stuff to hold things together), and a variety of fresh produce and a few odds and ends (an eggplant, cherry tomatoes, baby carrots, raisins, a gob of spanish moss, olives, etc.). Our challenge was to race to see who could make the best sculpture of a "middle aged goddess" from the things in the basket. It was hilarious, and the guests loved watching. I would think you could do the same thing with stuff from nature, pine cones, acorns, leaves, dried weeds, mini pumpkins or gourds, etc., then race to see who could make the scariest Halloween creature.