I also remember doing lots of stickers and sticker books when they were small. Of course, you run the risk of having stickers all over your car with this tactic. I used to open all the packs ahead of time and would remove the outline of the stickers in advance so that it was easier for their chubby little fingers to manipulate them (also great for their fine motor skills).
Now with their interest in movies and a nicer vehicle with a built in entertainment system rides have certainly gotten easier. I still only pack the LONG movies (over 90min) so we are not switching out DVDs every 30 seconds. I try to hit Target or Blockbuster before a big trip and get some cheap ($4.99 or less) new to them movies. Pack some snacks and instant movie theater!
Car BINGO - we just leave these cards in our seat pockets and the kids use them even on short trips!