Go to a "dagashiya" to buy some souvenirs!
Other than donating the change to a local or national charity, I recommend going to a dagashiya and buying candies and other sweets for your friends and family back home. Japanese sweets are incomparable to those of other countries due to its creativity and variety. Perhaps you should avoid sticking to just candy – go wild and choose something you’ve never heard before. How about Karamucho or Jyagariko? You may also want to go traditional and choose some mochis or senbei that you cannot find back home! These are usually very cheap and is a good way to use your change!