We drive up I-95 (1,200 miles each way) from our home in Florida to family in upstate NY several times a year. We carry more with them than we did for trips with our kids...but we stay for a time and need pens, beds, etc. The only thing that I would add for a day trip would be a classical music CD. There is research that classical music calms dogs, and all three of mine respond well if there is a traffic jam, too many trucks, or emergency vehicles. You can see their body language react! We always leave the classical music channel on when we leave them alone in the house too.
Have a wonderful time!
