According to Csikszentmihalyi, four components of an activity tend to make it enjoyable: we must feel like we are confronting tasks we have a chance of completing; we must be able to concentrate on what we are doing; there should be clear goals associated with the activity; and there should be immediate feedback provided. Activities that satisfy these conditions produce the sens of a deep and effortless involvement that takes us away from everyday life; give us a feeling of having control over our actions; take us away from our concerns for our self (which then emerge stronger after the experience is over); and alter our sense of time, such that hours can pass by in minutes. (notes via this review)

[Also, I have it on good authority that one can remember how to pronounce Csikszentmihalyi's name via the following mnemonic: imagine smoking a joint with a pretty girl: "Chick Sent Me Higha".]