This is more of an idea from my past that my mom used to keep me busy. I haven't tried it yet because it's a little bit over a 2-year-old's head. Anyway, my parents were the king and queen of roadtrips and they have instilled in me a love of the open (and long) road. When I was little my mom had my brother and I play Car Bingo. We had a ton of Bingo cards with pictures of things you might see on a trip - animals, road signs, and "special" cars like ambulances, taxis, buses, etc. Then with some simple dot stickers (like the kind you get to mark things at a yard sale) you're all set. You can make your own up or I found this website with a few choices for you to print out. There are more cool ideas and printables here.
Another little tip that I've found helpful is to buy a new toy/book (it doesn't have to be expensive) that your kid doesn't get to see until the trip has started. A friend helped me take this a step further (and keep things real on a tight budget). Go out and experience the wonder of a Dollar Store. Instead of one toy for $10, you can get 10. Then every so often you just introduce a new toy. And if/when the toy gets broken, lost, left behind, you're only out a buck. The last cross-country flight I went on with the Z-girl I got her silly putty, a few cars, army figures, Dora stickers, and pinwheels. It was great not to worry about losing anything in the plane or her crushing any of the toys.
If all else fails, pray they sleep! The Z-girl only sleeps 1-2 hours, no matter how long we're on the road so even that option is out for me. Oh well.