Hi D.,
Well I have not taken a long road trip with any child under 5 so for the baby I am not sure, however my family of 5 just took a long drive from Michigan to Florida a few months back (18 hrs.) my children are 16,10,6. The trip was great. To keep them busy we had a DVD player with all sorts of movies. I have an ipod video 30gb with movies, tv shows, music. My oldest son had his psp(playstation portable) which does all my ipod does plus games and he also had his mp3 player. My other son and daughter had their disney mix max media player which also plays videos, movies, music. Retail for $100 in stores but can get them new on ebay for $50 or less.
Plenty of snacks, and thats all it took. They enjoyed watching movies. You can get ones they have never seen, some favorites and some tv shows on dvd, and maybe since the baby is so young dvd's with sing along interactive.
Little Einstein's, The Wonder Pet's, Word Girl, Dora, Diego, Blue's Clues, Barney, etc...
Good luck on your trip.