Without sounding nosy, here's a rhetorical question ...are your
parents not able to increase their activity due to health issues or
are they just deconditioned? IF it is due to just being deconditioned,
there are trainers they could work with who will help them improve
their overall fitness level.
I'm wondering the same, Heather. It's not just the walking but the standing that factors into many tours: Rick's own combo tour of Venice/Florence/Rome requires that you can "Be on your feet, walking and standing, for up to three hours, indoors and outdoors, in all weather conditions." Even 1/2 day tours of some attractions, like the Vatican Museums or Colosseum/Forum/Palatine can involve 3+ hours of walking and standing with no place or time to sit. As mentioned, many city walking tours move at a pretty good clip, too. So, tours are not necessarily going to be a solution for their challenges. Neither, IMHO, are hoho buses in this particular country for all the reasons Roberto highlighted.
Traveling independently may be the best choice as it would allow them to move at their own pace. Many guide books, like Rick's, provide self-guided walking tours that would enable them to sit for awhile when the opportunities present themselves. Many audioguide tours are available too. My husband and I (70+) are indy travelers so far, can do hours/miles on our feet without much of a break, and very much enjoy spending as long, or as little, in one spot or another versus married to someone else's schedule. It takes doing quite a bit of pre-research to pick and choose our want-to-sees but we enjoy that.
I'll vote for Robert's plan: Venice for at least 3 nights; Florence for at least 3 nights; Rome for at least 4 nights; Amalfi Coast for at least 4 nights. I'll also caution them not to choose Positano as their base on the AC due to the amount of steps and inclines involved in getting around. As already mentioned, they need to choose hotels carefully in all locations to avoid having to lug bags up flights of stairs. Traveling with few bags (and those packed lightly) is strongly advised to make getting on/off trains and moving around with them as nimbly as possible: they need to be able to manage them themselves in many situations.