When we drove from Dallas to Orlando back in 2009, the DVD player was a lifesaver. I found some cheesy DVDs that were probably produced back in the early 90s that had the life-size characters from the parks at WDW. DD loved them. It showed different areas of WDW, so she got really excited about getting there. I found them for like $5 each at our
Walmart. Also brought our entire Disney DVD collection from home. I just put the DVDs in one of those CD wallet thingies (not the technical term, I know

).
I also bought a travel tray for DD to place across her lap so that she had a hard surface to color and draw on. I bought Disney themed window clings so that she could put them on and off the window however she wanted to. Kept her busy for hours. I found her some magnetic paper doll game where she could dress the doll in all these different outfits. She also enjoyed different colored pipecleaners.
I hit the dollar section at Target and the Dollar Tree.
Also, I know its not an activity, but since you have little ones, I HIGHLY recommend this....
Its a travel potty. This was probably the SINGLE greatest item that we bought for our car trip. DD had just been potty trained for about 3 months when we took that trip and it was a lifesaver! It saved the day several times on the way there and back when we were in the middle of nowhere and she had to go. The bags have a liner, so there is no mess. Just tie the bag up and dispose of it at the next stop. The potty is surprisingly durable. We still use ours now.