Hiking/walking/camping or anything with nature. Cleaning or yardwork is always a lot more enjoyable after smoking. Sometimes I enjoy exercising when I'm high, especially something lighter like stretching or tai chi, although motivating myself to exercise after I smoke can be hard.
If I'm feeling less active, movies, video games, or conversations with friends are always good (assuming you are alone maybe via text messages or internet). And of course music. Sometimes I like to just lie down with headphones and some good music (I find goa trance especially good when high) and zone into the tunes for a while.