We've been camping with our kids since they were weeks old, so it really depends on the age. My kids will find other kids to play with, and campground people are friendly, but I haven't really seen non-seasonal campers get involved with anything like a Halloween Boo. Check the campground's facebook page or call the office and ask if there's any traditions that you should know about (sometimes, there's Christmas week or Halloween week).
As for fun, these are things that our kids have enjoyed through the years:
Indestructable tonka trucks
Coloring Books
Water Color paint or the Just-add-water painting books
Army men
Bocce
Solo cups
Cards
Books
Football
Tennis balls
Frisbee
Ladder ball
Don't pack too much, but do have some things available. Don't put it all out at once. And, if it's something you like, and want to keep nice, don't take it. Make sure it can be hosed down or run through the dishwasher.
We have a no-electronics rule in the campground, including TV, although we do see many families who set up gigantic televisions. Our kids complain for about ten minutes, until they discover something like the solo cups and golf balls and make a game of it.
2 Parrotheads
2 Parakeets
2004 Trailblazer
1999 Coachmen Clipper 1260
"Come along, let's have some fun
The hard work has been done
We'll barrel roll into the sun
Just for starters"
-Jimmy Buffett