Colorforms.
Someone already mentioned a magnadoodle. Etchasketch is another thing which is awesome for long car rides.
Puzzles-- like Rubik's cubes, etc.
DK books are good because they have a LOT of information packed into captions-- so small
bits of reading, high interest level and something new to see/absorb pretty much every time you look. DD especially liked the more-science-oriented ones at that age, but interests vary. Personally, I thought that the one about string instruments was
awesome.
Best make whatever you provide something that YOU can stand to hear about for a hundred miles or so, though. Take my word. LOL. (Note to self; 'snakes' book = NOOOOO...
)
JMO, but I hate pencil games in a car. DD always managed to drop colored pencils/pens and lose parts trying to refill a mechanical at highway speed, and don't even get me started on what a pain
pencil shavings are in upholstery. DD has never really gone in for gaming in the car, though she does enjoy her DS for extended trips. (I completely understand not wanting to LISTEN to the darned thing for hours, though...) With younger kids, if you can stand the stench, the Crayola color-wonder products are nice (well, DD liked them when she was.... okay, like two). Sounds like your two are a bit past that point.
Maze/activity books are good for some kids. My DD never was all that fond of crosswords or mazes, but liked some of the other types. Usborne and Dover both have inexpensive examples. There was a brand of these that we used with DD when she was your DS' age... gosh, I wish I could remember the brand. It was quite distinctive-- they were about 6" square and came a great variety. Baku? Boku? Something like that. They were (IMO) much more absorbing than many in this genre. There were the 'hidden picture' variety, which were revealed only with a pencilling-over and then one had to locate the 'hidden' objects (much like a Highlights version of same)... there were puzzles, word-searches, even some color-and-cut models and playsets (I remember that one from a hotel room where DD set up her "farm" over a period of a couple of days). DD really loved those for travel and my office/meetings.
Sudoku/Solitaire hand-held LCD games are a good time-killer, too, and they're quite reasonably priced ($3-10 usually).
A lap desk of some kind is appreciated by some kids-- a magnadoodle or colorforms board can work as one. DD likes to use hers to lay out a full Bento-style lunch and play with her food a bit.
(We've done this kind of trip since DD was about 1yo, and given that family has lived between 400-800 miles from us through.... er... less-than-scenic terrain... let's just say that we've got a routine.)
Things that work for two kids but not one:
Mad libs
joke books
riddle books
bingo-style games
A dry erase board may be fun, too-- just be aware that you'll need IP alcohol or acetone to manage any accidents with the pens.
Bottom line is that some kids are just much better about car travel than others. We were incredibly fortunate to have a kid that doesn't mind long car trips. She's a trooper, even at 14-16+ hours straight through.
We tire long before she does.