Csikszentmihalyi says that what makes people happy are activities which have (a) clear goals, (b) clear rules, (c) clear challenges that are neither too difficult (leading to frustration) nor too easy (leading to boredom).

I've found this idea really useful in teaching. (I know, you're shocked.) I first encountered it in James Gee's What Video Games Have to Teach Us About Learning and Literacy; part of Gee's point is that a well-designed game provides exactly that sweet-spot experience of pleasantly frustrating, challenging-but-not-impossible activity -- I think he also calls it "edge of ability," though I forget whether that's in this book or elsewhere -- in a way that school frequently does not. Which... yeah.