Twizzlers. They saved me while taking an 8 hour car trip with my kids by myself. I also had some drinkable yogurts (not gogurts, but the cup kind) and cheese and summer sausage. String cheese is a hit and can take a few minutes to eat. Also the large pretzels are not so messy.
As for toys, balloons worked really well when we took them to Tennessee last year (20 hours one way). I found a pack of balloons and a pump at Target and it took up a lot of time. they were messy - whenever we opened a door, they would fly out, but they took up a lot of time!