I was on the same cruise.
I think future cruises have less Japanese ports in the itinerary which is a pity - JMO.
Petropovlovsk was cold - river still frozen in the town centre and it was May! I do cold btw - I am Scots.There were tendering issues and not all the passengers made it ashore. That port is once and done.
The activity team and the CD did themselves proud with a very crammed schedule.
I travel solo and if there were solo activities, I don't recall them. That said, it was no hardship and I and a few others formed a very fun international group at the Martini Bar. Great time was had and we are all still in touch. We formed our alternative choir.
A lady organised a table for solos through the roll call - I would not do that again. However that is possibly one route if you wish dining companions.
I thoroughly enjoyed my cruise which was greatly enhanced by the 'Martini Clubbers' and the 10 days land trip I spent crossing Japan prior to the cruise. I plan to repeat the land portion next year but will not repeat the cruise - frankly there are too few Japanese ports.