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I managed round Pompeii with my son who was 4 at the time and we had a tour guide. There is a lot of walking and it can be very hot but I wouldn't say it was strenuous.
I think that if you take moderate ones to be our fairly mid you'll be ok. The strenuous ones are usually for the fit so leave those alone but there is an over-exaggeration with these categories mainly so that there is no comeback afterwards. Pompeii does involve a few slopes and hillocks and we had a guide who was on a promise because he was in a hurry and wanted everyone to keep up but he was the exception rather than the rule. so go. It was less strenuous than Herculaneum but Herculaneum was more interesting than Pompeii. The Acropolis in Athens is up a whacking great hill but shouldn't be missed and if you are unable to cope with the climb up there is a lift for disabled people around the side but you must be accompanied to use it.... Neil.
Athens a great place .. a visit to the Acropolis is a must but as Neil said it has a slope and some steps .. also make a point of having a stroll around Plaka area. To see the real Athens it's a place to walk around .. mind you I am biased as I have been visiting Athens and Greece for nearly 50 years. Have fun .. Neil.