Number 2 .... this is the really important one
Have snacks. Maybe it's just the Mom in me from having littles, but snacks are so important on any road trip. Now that the kids are older it's easier for sure, but I am still picky about what they eat in the car - crumbs are a pain and I'm the one who has to vacuum my car. We try to stay away from sticky things, we have wipes, but it's just easier to stick with the things that aren't sticky. Jerky, crackers, popcorn, granola bars, grapes, things along that route are typically what I pack in the car still.
Along with the snacks everyone needs a reusable water bottle. I swear we have 50 of them, we might as well use them, plus it's easier to refill them as we go than to buy a new one.
It doesn't matter if it's a day trip or a week long trip - you still need snacks.