There are effectively places to sit every few feet of MK, so I'm not sure it makes sense to list all the available options.
There are many park benches and every quick serve dining has tables nearby. So in one sense, you'll have no trouble finding a few spots to meet your needs.
Any of the shows are going to have seats, but they are not exactly a quiet place to relax. They are pretty much all rather loud. Waits are usually short, but some shows only run on the half hour, so guests might have to wait - STAND - up to half an hour before they can enter the theater to sit. Some shorter shows are MonstersINC, and Philarmagic (in middle of MK), but guests have to exit when the show ends.
If endurance is an issue, it might make sense to rent a
scooter for the day.
Having gone to WDW with extended family, I don't mean to negate what you are trying to do, but my relatives usually just reached a point where they were done
with the park, and wanted to leave for the comfort and quiet of their hotel room.
Part of it is the noise factor. Almost no matter where you go in MK, there will be noise. There's also always going to be the dreaded walk out of MK, which is going to be long if you are offsite. Another common reason they may prefer the hotel room is ahem- because the hotel has a restroom a few feet from the bed.
In my family, I also hate to add....spending time alone isn't something they were eager to do on a family vacation, especially if they are not familiar with the park Including them usually means we have to travel at a much slower pace- period. We also tend to stick more with the slower attractions. They maybe are willing to babysit a small child - as in take the small child on something like the TTA while everyone else goes on Space Mtn with their fastpass. But that often kind of marks the end of their day. I find endurance improves the more they are included in what we are doing.
So the most successful strategy we have found is to include breaks in our day. See shows often, and stop for little sit down snacks. We also pick TS places that have minimal wait. You'll also do well to avoid any doubling back.
Going over to the Grand Floridian has been one place we sometimes can take an afternoon break that is fun for everyone. It isn't quiet, because they have live musicians much of the day.(including a pianist), but it is pretty and enjoyable.
