When travelling light, it's essential to pack compact, and to keep all your gear in great condition. Personally, I keep a set each for both the winter and the summer.
The quintessential on-the-go entertainment. Great for killing time and getting to know a person. You can learn a lot about someone by just playing a single game of cards with them. Plus, it can be a fantastic conversation starter.
I find playing an instrument to be incredibly relaxing, and a great way to wind down. Bonus, it's great for entertaining friends, and new acquaintances. While I adore the piano, I have yet to find a portable digital piano to take with me. For the time being, a guitar will do. I'd like to eventually get a panjuri, as it's even more compact, and I'm more used to the three-string arrangement, since I played a guitar dulcimer for quite a while.
If you're going to be travelling for a long time - or even if you're staying at home, I strongly recommend that you get yourself some sort of instrument. Great for amusing the lasses, too. ;)
Perfect for bird-watching, amateur astronomy, and plane/satellite-tracking.