When visiting Japan, if you have any intention of using public transit at all, get a Passmo card! This card can be loaded with money and used to take metro, buses, and can even be used in many arcades and at some vending machines. In a country that has a largely cash based economy, this card will keep you from carrying too much change.
What the Passmo card is best for is taking the trains around Tokyo. You could show up at the station, plan your stops and figure out which trains you need, enter this into the ticket machine and break a bill or count change to purchase a ticket, or you could scan your Passmo to get to your platform and scan it again when you leave. The Passmo is also much faster and may make the difference of catching your next train or not. While the Passmo card is convenient, it also is often cheaper than purchasing tickets.
Trust me, the Passmo card takes a lot of stress out of using the public transit in Tokyo which can already be a bit intimidating as you are pushed into a crowded car.