For us these are some things we've taken:
1. anything he sleeps with or uses as part of the bedtime routine: blanket, pillow, stuffed animal, toys, books, etc. My DS sleeps with numerous pillows but is allowed to bring only one (of his choice).
2. Toys he plays with regularly that are appropriate (not too big or noisy!) When my child was little, this was a small wooden train set that he played with daily for years. Now it's
Lego!
3. Snacks and drinks that he doesn't know about! If he knows about them, this is all he talks about and pleads for until they are gone!
4. A surprise book about his special interest. I will give this out when things aren't going well and it usually saves the day!
5. A camera! At first it was a disposable one, now it's an inexpensive 35mm film camera. This also helps them after the trip if they tell friends and relatives about their pictures. One trip my DS was fascinated by palm trees and almost every picture was of a tree from underneath it! Took me a while to figure out what those green blobs where when I got the film developed!
6. A surprise bag. This was a small lunch bag with little surprises like a small pack of crackers or nuts (age appropriate of course), a very small amount of candy, some small toy, and activity books (mazes, word search, coloring, etc). These were only given on days when we drove long distances and given out partway into the day when kids were getting bored and noisy. I keep my eye open all year long looking for small inexpensive items.
7. A backpack. Now that he is older, we use a backpack which he fills and we have to approve of the items in it. He also uses this to keep track of anything we buy or give him during the trip.
That's all I can think of right now. We just got back from a plane trip yesterday, so some of this was very fresh in my mind!! Food was a very big issure with him this trip, he would be hungry 15 minutes after we ate, even though he is a skinny kid!