Bleh. I hate wasting time.

So, what can I do for about four hours off and on that is:

1. Fairly easy, doesn't require too much brainpower
2. Productive
3. Entertaining
4. Will give me some sort of feeling of accomplishment afterwards

Any suggestions?

[Edit: I'm thinking something creative that preferably doesn't involve cooking, and my flat is spotless, so cleaning makes no sense.]