J.✰.
If you can find christmas lights (or fall-themed lights) to plug in to your cigarette lighter in your van, that'd be fun to string from the trunk of the van (once opened).
Glow-sticks/necklaces might be fun for the kids.
You'll want plenty of ice in your cooler with drinks for the adults AND kids. If there's tea or plastic cups, it's always nice to have a Sharpie on hand to write names on.
Boom box with music? David Crowder or Jeremy Riddle my fav.
Foldable chairs.
Recipe? Is there a theme? If it were me I'd do chili or somethingthat stays warm easily (crock pot, camping stove). I am not a griller.
Have fun!