I fly somewhere between 340-400K KMS each year, and that (obviously) implies spending hours on planes.
Although I tend to work one way, often catching up on the deluge of eMails that piles up in my inbox, I try to take some time and recharge on the way back.
In my case, this involves writing music. For me, it's not only a great way to stimulate a completely different part of my brain, there is something about the process of creation that is all consuming and makes time pass very quickly.
I carry a very smart little keyboard from CME called the xKEY. It is low profile, fits in any knapsack or shoulder bag alongside a laptop, and yet is velocity sensitive, complete with after touch. In fact, all you need is an xKey, a Laptop, and your favourite DAW, and you have a fully functional recording studio at 40,000 feet.