What is the monk's biggest hobby?

You can find the answer to this in Chaucer's General Prologue. Look at lines 165-207, in which the narrator describes the monk. Unlike many other monks, who would spend their time bound to their cell or monastery as recommended by most holy guidelines, this particular monk loved "pryking and..hunting." In this context, "pryking" means "riding." The monk enjoys "venerie," or hunting. This seems to go along with his robust and masculine personality.
The monk explains to the narrator that he does not see why he should remain passive and bound to his cell when he could be out riding in the world, and the narrator agrees that he doesn't see how the world would be better served if the monk did not pursue his "lust," his hobby. This riding monk does seem to have some other habits which are also a little unbecoming to a monk, however. He spends a great deal of money on his fine garments for riding and hunting in, and he is fat, suggesting that he does not restrict himself in his diet either.