What sort of backpack/ luggage should I use for a 3 week trip to Japan? -

I will be in Japan this December, starting in Tokyo and more than likely traveling to a few other cities. I want to pack as light as possible to avoid having to cart big luggage with me onto trains and subways. Thanks in advance.

I m having the same trouble I ll be going to Germany for 3 weeks in July. I might just get a medium sized suitcase with a backpack.