It depends what your interests are. I have a lot of dvd's to watch, some games if I get desperate. I also do plenty of reading and make sure I always have a couple of books ahead of the one I'm reading waiting for me. I'm a member of a few online communities so I can immerse myself in those for the duration too. If I'm more energetic and enthusiastic I have a few projects set aside that I 'never really had the time for' before so sometimes with all the extra time I have I get to get stuck into them.
Exercising is good too, not just for the time you're exercising but if you do a good workout then the time you're recovering from the workout is also time you're not thinking of things you shouldn't be, it usually also helps me sleep better at night having done something physical during the day.
The important thing is to have many different activities planned because you never know what your energy or enthusiasm levels are going to be.