Thanks, this helps. They are 78, just had their 56th anniversary. Dad likes to use a big walking stick, like he is some kind of shepherd lost at WalMart. Rough streets would be tough for him. Mom could probably manage OK.How old are your folks? We're 71 and have our own health issues. How much walking you do depends on the shore excursions you book. Some cruise lines offer indications of how much effort is required on a given side trip (easy, moderate, strenuous) so you can use those guidelines.
Remember that many streets over there are cobblestone or the like, and sidewalks are not exactly even. If your folks use canes, make sure the canes have a wide base on them to keep from slipping. You can also choose to take a cab to a specific sight you want to see in a town...the ship concierge can often help you with procuring a cab, We found Uber to be iffy at best.
Also, river cruises tend to market toward older folks Many of them require passengers to be at least 18, and on the cruises we went on, most were 40, going clear up to mid 80s. On a river cruise, your folks might enjoy just sitting and watching the shores go past.
My biggest challenge might be mom would want to go, but dad wouldn't...