I'm not sure why you're dismissing building character as an explanation. Something builds character if it helps a person develop virtues such as patience, perseverance, humility, temperance, etc. Committing to a difficult activity can obviously do this, perhaps more so if it is not instrumental. There's also the sense in which an activity can be a test of character, so that completing it reveals (to oneself and others) virtues (or room for improvement). I find "direct hedonic value" far more suspicious, since most "rewarding" activities offer nothing of the sort (the pleasure of a difficult activity is usually delayed and the activity itself may be wholly negative), and those that do are usually considered addictions/vices/etc. Hedonic reward also has only a tenuous connection to an activity being considered "interesting"; in fact, probably only boredom can stop an activity from being interesting and I'm not even sure about that (in many endurance sports the tedium is part of the challenge).