A recent article in Travel & Leisure recommended travelling with a tennis ball in your carry on bag next time you take a flight. It's good for giving yourself a massage when you're sitting down. Which is extremely important, because sitting hours on end is bad for your back and legs. It also increases blood flow, which can help with jet lag. Just put it behind your back, lean on it, and roll it back and forth. Or you can massage yourself by rolling it across your shoulders, legs, and feet (just make sure you have socks on lol).