Like Lisa, we've done this at church but not school. How about each "trunk" devises a game stationed in their parking spot. We've also had a table with Chili and Hot chocolate set up.
As part of our Fall Fest, we do "booths" with nail painting, rub on tattoos, glitter hair sprays, and face painting. This helps keep the kiddos entertained while the lines for our jumping castles aren't nearly as long!
We have never done it at school, but I have done this at church. How about a few contests? See who can "spook" up their car the best. Best costume, scariest, there are a lot of things you can do. Have some kind of arch or tunnel for the cars to go under when they are parking. Don't forget a few games to entertain the kids, bobbing for apples, keep it simple though. I tend to get wild with my imagination. GOOD LUCK!
We will be having trunk or treat in mid-october, where the kids get to trick or treat from the different cars. We are looking for some new fun ways to get the whole family involved. Any ideas?