It's of course a good idea using a private guide in Athens, especially regarding your 84er parent's needs.
I just want to tell you that we toured Athens by ourselves without any problem: my wife, me and our two teen sons (13 + 15) found it amazing to ride the town by subway, bus and going on feet.
It's easy and cheap to reach Acropolis: there's a subway station just a few steps from it. Many signals are both in greek and english.
Under the turkish domination the town had become nothing but a small village: it has been reprojected by german architects after the falling of the Ottoman empire so you will see, actually, something like a 19th century central-european capital.
We read a guide book before departure and studied our tour; we made even some "real life" in Athens, going to markets, shops, bars, mail office to buy stamps and post directly our postcards ... we took a lot of photos, had a great time
and then returned on the ship in perfect time.
I'm sure you will however