Not really a toy, but what my daughter likes best when we fly are the snack packs the airline gives out. When I flew alone with her from DC to Las Vegas, she was awake the whole way there and back, and kept herself amused with a pack of peanut butter crackers that Southwest gave out as a snack. She likes anything crinkly, but only real crinkly (I've tried getting crinkly toys and she could care less). So I kept hte peanut butter crackers and they will be accompanying us on our flight to Disney this Friday. Also, I don't care if she throws it on the ground or anything like that since it's just a pack of crackers, not a toy I bought.
As far as real toys goes, right now (she's 9 months) she likes anything musical, including stuffed animals that talk/sing when you squeeze them. The only problem is that she can't sqeeze them herself yet so I have to do it over and over. Not bad on a plane, but a little inconvienent when I'm eating dinner.