I was on 12 night Seoul to Tokyo cruise but on the Celebrity Millennium just last month. We had an amazing time . Most of the time we got into port around 7-9AM and departed between 4-6pm. Most ports I found that adequate to see what I wanted to see. The ship stayed overnight in Osaka arriving at 9AM the first day and departing at 8PM the following. Because my main focus was Kyoto and I booked an early morning tour of 3 of the main sites so the second day we choose to stay at a hotel overnight in Kyoto . We also had a tea ceremony and tickets for Ninentdo museum there. If I were to go back I would have loved more time there but we did most of the things we wanted those days and got back to the ship with time to spare. It was our first time using the train system which was a bit of an adventure but google map was great and we didn’t get lost.
I did add 3 days prior in Seoul ( wish I had done one or two more) and 6 nights following in Tokyo ( last 3 were Disney).
I found that prioritizing the things you want to see the most and not expecting to see everything worked best. I ended up cutting out a few things due to time, the heat (humidity

), some very rainy days and exhausting/ sore feet but I don’t feel like I missed out as we had a wonderful trip.