everyone's suggestions are perfect. We do a few 10-hr and 13-hr trips a few times a year (10 hours back "home" where our parents are, and 13 hours to where my FILs family is and where hubby used to live), and our must haves are ...
*snacks, lots and lots of snacks. lol
*portable DVD player and 3 DVDs picked out by each kid (there are four of them after all. lol)
*portable gaming devices (PSP and GB Advance) and all the games that we own for each.
*special car-only activity sets purchased by my clever brother (aqua doodle, coloring packs, uno travel game, cards, etc)
*books, books, books, books!
*cars for the boys, Bratz for the girl
*travel pillows and their special blankets
I am really lucky - ALL my children travel well, as we've always done long car trips. We only stop when we need food or gas, and they never complain. None have ever gotten car sick, but we always bring an old ice cream pail and keep on the floor in the back seat just in case.
The other thing we have started doing as they have gotten older? Before we leave, the two oldest ones get a notebook, and in that notebook we ask them to find 20 things along the trip ... like pink car, purple house, birdsnest, hat on a fencepost ... they love it!