HI! Janet here, with some details about Aegina, an ideal day-trip from Athens, even with limited mobility. I answer this query so many times, I'll just cut & paste one response, tailoring it a bit --
To show you the simple pleasures of this outing, Here's a photo album of a june daytrip: http://www.flickr.com/photos/36264706@N03/sets/72157621604646139/detail/ - so much to see... and the short voyage there there is half the fun! Only challenge at all - to get to Pireaus in time for an 8:30 sailing.
The KEY to real enjoyment is the right ferry; NOT A hydrofoil (small - enclosed - salt-rimed windows - no view).. instead use "COnventional Ferry" (I.e. open deck); you can sit out and enjoy a 1-hr "mini-cruise" ... watching the wake, seeng everything from giant cruise ships & freighters to 3-masted schooners, speedboats, and tiny sloops.. maybe even a dolphin. These ferries go less frequently - first is ?8:30 AM, next not til Noon. THe Green Line Metro is a quick trip (20-30 mins from Monastiraki station) but that involves lots of stairs/escalators ... so you're better off taking a taxi. Since it's in rush hour, it may be between €20-25 -- a "street taxi" might be iffy on price so have your hotel call ahead for City Cab or Welcome Pick-Ups (u know fare in advance, pay by credit card)... In fact, use this for most trips. These cabs will take you right to ramp of the ferry. PS you can buy ticket ahead, at any ticket agency by your hotel.
Aegina has a charming port town complete with small ruin right by the ferry with ONE remaining column (have Mellissa pose by it with sea background - makes a great photo!) A TINy museum next door mainly notable for huuuge man-size vases. The main classic sight is stunning Temple of Aphaia mid-island, on a hill (not steep) with 360° view of Saronic bay. I usually recommend bus + hike, but for you, a taxi. At taxi stand, agree on price for the following -- drive to Monastery (seen in video) pause for a look -- continue to TEmple & wait for 15 minutes while you walk around it & take pics -- continue to Ag. Marina, pleasant seaside town. There you can loll at a waterside cafe and have a nice lunch. Staff can tell you time of next bus back to the port, or call a taxi.
Back at port town, if energy permits, it's fun to walk through back lanes... Aegina is not a "foreign tourist" target, so its back streets are Greek... I love to browse local grocery/hardware stores for products. Relax at a portside cafe with a drink and enjoy sunset on the last open-deck ferry, sailing at 6:15 -- or if you've had your mini-cruise in the AM, you can get a Hydrofoil earlier & reach hotel for a rest before dinner.
PS: Regarding solutions to other mobility situations, I've sent you a PM; look up in RH corner of this window.