I presume your question is mostly about what to do in the hotel itself (maybe in the hotel room itself) for a situation where you are away A LOT. In this case, it's not a "night away" or down time, it's more like the millionth night in a another boring hotel room. My suggestion is to develop some basic habits other than crashing, watching TV, surfing the net, and other stuff that feels like wasting time. Planning to exercise is a rather obvious one. It's good for you, energizes you, and can be done anywhere. Walk around the neighborhood or a historical part of town, do P90X in the room, etc. when the hotel gym is not appealing (or get a day pass at a local gym).
Some less obvious plans might include:
- writing a book,
- learning a new language (e.g., get Rosetta Stone and watch the foreign language channels on the TV),
- professional development (e.g., get an online degree from somewhere or just learn new tricks),
- professional networking,
- vacation planning,
- design a new house or home improvements,
- find a new computer "hobby" (e.g., learn to edit photos or movies), or
- give yourself home tasks that you can do on the road (e.g., organize photos or files from your home computer, work on financial/investment/retirement plans).
Just in case you were not talking about just room activities, I cannot recommend enough that you get out and see things, meet people, and experience local cuisine and culture. You should also try things you cannot do in you own town. For example, a person from Oklahoma may not get too many chances to go sea kayaking.