I'm currently on a plane on my way to Tokyo for a week-long trip there followed by a week-long trip to China. Sitting here, it's obvious what a useful environment for language learning your flying time can be; the by-necessity multilingual environment of a flight to another language zone means you can easily get some exposure to your target language while in transit that probably isn't always readily available outside of where the language is spoken.
The first opportunity you'll come across are the flight attendants, who will probably be there to greet you when you get on board. Greet them in your target language, and if you've got any questions, try to ask them in the target language. They'll probably initially make assumptions about what language you speak. On a flight like mine from New York to Tokyo, Japanese-looking Asians will be initially addressed in Japanese and everyone else will initially be addressed in English, if the flight attendants have the language skills. I'm on a Japan Airlines flight, so most of the flight attendants are Japanese but of course all of them also speak English. Before I said anything to them, they predictably spoke to me in English, but once I spoke to them in Japanese they switched to Japanese. Some of them seem to be prefer Japanese when possible, and I'm more than happy to accommodate. You'll sometimes find that even if you speak your target language, they'll still won't use it back to you, but be stubborn; don't switch back just because they don't deem you target language worthy. Some flights will not have so many target language speakers, such as my typical Continental flights to Japan. In that case, you can direct any questions you have to those who speak the target language.
Although you're more likely to need to communicate in some way with the flight attendants, your fellow passengers are another good source for target language exposure. There's probably a pretty good chance that the person sitting next to you is a target language speaker. Find a way to strike up a conversation. Unfortunately, the person I'm sitting next to right now is a bit reticent and has been sleeping most the time, but on other flights I've had great conversations with those sitting next to me.
When they come around with reading material, grab something in the target language. Even if you're not that strong in the target language, get a newspaper or the like and see what you can understand. A few words here and a few words there might be all you can get, but it's nevertheless more exposure. There's also some reading material in the seat pocket in front of you and on signs around the plane. Although safety instructions and the like might not be the most exciting things in the world, they're usually in the target language and one or more other languages, so you've probably got a translation already sitting right in front of you.
The onboard entertainment is another good source. It will typically be in multiple languages, either with target language audio or with target language subtitles. Watch some of these to see what you can pick up, or if you're getting it all just sit back and enjoy.
And, of course, don't forget to be a good little Boy Scout and "be prepared". With a laptop, iPod, electronic dictionary, and an array of other possibilities, you can make that "wasted" flight time quite productive.