Agreed on Blue Lagoon. It exceeded what we thought it would be and we learned that sea lions have really, really bad breath.
Shore days can be a huge variety in duration. Our upcoming Northern Europe cruise spends only a few hours in Estonia but two days in St.Petersburg, Russia. My suggestion is to at least get off the boat and wander the portside community a little bit (unless there is NOTHING there) and if you want to, you can get right back on the ship. We were a little timid on our first cruise about going ashore since it can be intimidating, especially in a foreign/non-English speaking country, but keep in mind that there's a lot of other tourists around and the locals are pretty used to us.
On the plus side of 'staying on-board'- you'll have the pools, Aqua-Duck, movies, etc. pretty much to yourself and it's very quiet (relatively speaking).
I get the fact that there's a lot to do on board, but everything you list- wine tasting, spa, etc, can be 'done' at home. Going to other countries...can't.