The most popular ways of spending free time in Japan are: drinking parties, karaoke and golf. So even if you are not a top rank golf player and have no singing skills you can still concentrate on drinking. In the case you wish to have your life in Japan even more eventful I can suggest you some other activities, eg.:

feeding wild rabbits in Komaba park (Tokyo) ...	or deers in the streets of Nara, ...
watching sumo exercises ...	or practicing sumo yourself (I suppose, I do not need to mention that in such a case picking appropriate opponent is crucial), ...
	climbing mountains and feeling great when over the clouds, ...
	watching some festivals, ...
creating a perfect rock garden, ...	planting rice in your field, ...
going for a walk with your pet, ...	painting Japanese dolls, ...
going fishing ...	or going to China town when you feel you really need to change your environment.