Hello Frangipanni,
I ordered both the Yen and the Won for Korea. I watched the exchange rate and I believe I ended up ordering around 1200 USD worth of Yen (two people) as we were told Japan is mostly a cash society, which for the most part it is (we did not take any ship tours, but arranged our own). Many restaurants we ate at in Japan and Korea only accepted cash. However a few of the larger places we visited such as some caves and Tokyo Disneyland did take credit cards.
Did not take a tour in Busan it was on our own. My adult son got off the ship with friends we made on board and I remained on board as the air pollution there that day was really bad.
Hope you have a wonderful trip and see a lot of the Sakura! That was the most beautiful sight while we were there! I would love to go back. One of these days I will. 🙂