There are a lot of nice things to do around Yokohama for a few hours with luggage storage. I would also like to add Manyo Club which is a large onsen/sento where you can store your luggage and enjoy hot baths, jacuzzis, heated stone lounges, relaxing loungers that lie flat, etc. We are choosing that option for the morning because I often don't get enough sleep on the ship the night before, so this way we can relax and I can snooze a little more if I need to.