A 20 day cruise must have a ton of port stops. To add a few thoughts. Personal fitness level of all has to be taken into account.
Some ports are much simpler then others. Barcelona-the entire city, museums, eateries, even getting into the country side. Simple.
Piraeus into Athens and all the major sites. Simple.
Civitavechia train to Rome and tour the major sites, moderate difficulty.
Kusadasi and diy public mini bus to Ephesus and then to Selkuk for lunch and back to ship. NOT EZ. But doable with all the info on this board and the help of all these great ppl.
I would diy major cities to start. Builds confidence. Download self guides for museums. (Nothing beats a good local guide if money is not an issue)
I have followed the recommendations and instructions of Hlitner and Euro-cruiser on many trips. And have had amazing days. U just have to enjoy doing a ton of leg work.