Thanks for helpful replies.
First off I made a mistake, the first day is just 2 ships in port because Holland America is listed twice: once the 14 day roundtrip from Rotterdam and once the 32 day trip with extension to Boston via Iceland and Greenland. Note that contrary to above this ship does spend the night in Stockholm arriving the day before. The other ship arrives at 8 am and leaves at 6 pm.
I do recognize that water is the best vantage point for some places like SE Alaska, parts of Norway, the Galapagos, Antarctica, and other places.
I have seen places where cruise ship tour arrivals change the experience drastically. Last December we stayed at Uxmal, the Mayan ruin in the Yucatan, so we could have a quiet morning there before the tour buses from Merida arrived. We did and it's vital in appreciating such places to have times with tranquility. Later in the morning it became busier, then around noon we were ready to depart but I ran around re-taking some pictures with better light. Then the buses from the cruise ship arrived https://www.shoreexcursioneer.com/progreso/uxmal-mayan-ruins.html, who even knew that cruising from Texas or Mobile or New Orleans was now a big deal? Since Uxmal is out of reach of daytrippers from Cancun or Playa del Carmen I thought a major influx impossible. The place became swamped and circus-like with many loud, heavy, scantily dressed people posing for pictures in front of structures and it was a relief to get out of there.
Another time sitting in wonderful Old San Juan, really one of the great sights of the western hemisphere, enjoying a sunset by the water next to 2 enormous cruise ships, and marveling at their size. In less than the time it takes to eat an ice cream cone, the debarkation began and in just seconds the peace turned into a cacophony of teeming masses, like when a person kicks a log and it becomes black with ants immediately: identical, nameless, faceless hordes with tour directors and bus drivers and taxis screaming for passengers and, ice cream cones still unfinished, we ran.
Bottom line for Stockholm: the old town is also part of the cruise sphere of interest so first day go someplace less popular, maybe that royal palace on the island west of the city, or a day trip to Upsala.
Also thanks for the June 21 reminder, will plan around that, maybe hiking or visiting people.