Obviously it's not close to Athens, but I would strongly recommend a visit to the Meteors - Greek Orthodox monasteries built on tufa rocks - near Kalambaka in central Greece, near Larissa. They are out of this world, and are reached by steps from adjacent car parks. I have never been anywhere like them - lost in the mists of time and very peaceful despite all the tourists. Vaarlam and Grand Megatron are the most amazing places, but if you have mobility problems the nunnery (St Stephen's) is probably the easiest to reach on level ground. It's somewhere you could never replicate, even in St Peter's in Rome or the Dome of the Rock in Jerusalem. Please take the risk and the somewhat complicated journey and have an out of this world experience.