Car's stuff has been hit and miss and finding stuff at Wal-Mart. I haven't looked lately, but if they have it in stock, you can get...
Hats, plates, napkins, table cloths, party favors, etc.
But I've also seen some "Cars" drinks. The are in those little aluminum cans (8oz?). The Lighting McQueen drink is fruit punch, the Sally drink is fake lemonaid.
We had a "Cars" party for my 4yo in August. A few people decided we had gone a bit over the top when one of the kids fell and needed a bandaid... and we pulled out a box of "Cars" bandaids.
I don't know if things are going to get better when the movie is released on DVD, but the "Cars" toys are a lot more difficult to find than they used to be.
Basically, just keep your eyes open when ever you go shopping for the next month.
If the party is AFTER the DVD release (the 7th of November I think), perhaps their will be more merchandising to go with it.
Otherwise, I thing you are going to have a problem doing things out of the ordinary because "Cars" is a new set of characters so there isn't much other than the "ordinary" in terms of merchandise out there. Obviously you could do anything that involves cars, but then it's not really "Cars" when you do that. The only exception would be if you or a friend were artistically inclinded and have custom make something.
For the first few months after the movie release, it was easy to