K.M.
We drive to Destin almost every year and have since our kids were two and four. It is an 18-19 hour drive. I STRONGLY recommend that you break this drive up into two days. We would take the first day and drive until we couldn't take it anymore (10-12 hours) and pull over for a good sleep, eat a good breakfast and continue our journey. We arrived rested and ready to play!
That said, the Dollar Store is your friend. Find some cheap toys, coloring books and crayons. They also have some "modeling clay" (not play doh) that the kids can squeeze and mold without making too much of a mess in the car.
I also recommend TV trays, or lap trays. I found some years ago at Wal-Mart that were metal and had short, fold down legs and they fit right over the car seats. They were awesome. It made eating meals and snacks so much easier and drawing/coloring/clay time was much more fun. Just remember to bring wipes to clean them up.
Other must haves for the car: baby wipes for cleanup, water, snacks, extra batteries for toys, keep an extra change of clothes handy for any accidents, and lots and lots of patience.
Yes, the drive is long, but we have had some of our best memories in the car and it really brings us together as a family. You will have a wonderful time. Good luck!