I spend a lot of time in used bookstores and one of the things I look for are lightweight paperback books suitable for backpacking. I try to have a selection of 6 or 8 titles set aside that I can choose from, depending on what sounds interesting. Average weight: about 4.5 oz. That's my main in-tent entertainment.

I also own a 1 oz. mp3 player w/ 2GB of music that I can take if I wish. Sometimes I take a small notebook in which I can write notes about what I've seen or thought about on the hike. I might spend a few minutes reviewing the photos I've taken, but that's a pretty minor pastime in terms of how long it occupies me, usually no more than 10 or 12 minutes at most.