I have a 2 year old daughter that has been a big learning experience with long drives, flights and trips. A couple of things that have helped my wife and I from going nuts:
Portable DVD players-with all their favorite movies, and some they have not seen yet. My daughter goes nuts for Elmo, Caillou and Sesame street. This alone can keep her content for hours on end.
If you have a smart phone like a Iphone or Android, get some of those shows on your phone. Helps when you are waiting at restaurants and in situations where you cant always have the DVD player.
Small toys and interesting objects are important too. But pace yourself on how and when you introduce them to the kid. You dont want to use up all your resources too early. Like we know what her favorite toys are, but time interested in things she already has does not last long, so have new things, like a couple small toys she has never seen. It does not have to be fancy, my daughter was fascinated with my watch for a good 20 minutes one day.
Snacks are super important too, my toddler gets hungry often, and often at very inconvenient times. So crackers, and welches gummy fruits are always good. Easy things for that at airports and decent truck stops are the small fruit containers and crackers and cheese. (cut to non-choking size, you will need to do) Blueberrys are pretty safe.
As noted above with the toys and movers, whatever you do, dont let them pack the kids favorite toys! You will hear about it for days!
Crayons and coloring books are great too. Keep a smaller notepad and a small box of crayons in your bag, or wifes purse, they help out a lot, and having a back up for long stops or dinner on the road is awesome.
One more thing, for little ones have more than enough diapers, wipes, and changes of clothes. Its easy to over look how many diapers the little one might go through in a day.
I am no expert, but I am learning as I go with this stuff too, I hope these things help you out. Good luck on your trip!