Here are my pieces of information on Aegina, which I keep posting, like a broken record:
• Photo album of daytrip -- http://www.flickr.com/photos/36264706@N03/sets/72157621604646139/detail/ it shows the "mini-cruise" of an hour you can get on an open-decks ferry ... the visitor used a car, but it shows highlights nicely, and metro station at Piraeus.
• Do Salnav advises (I know her, and confirm that she Knows All) about buying tickets in advance ... how far, depends on your planned date -- The Holiday itself is Sat Aug 15, but the week before/week after may be quite busy -- and weekENDS are a no-no.. boats AND island will be jammed. For a single day, always choose a weekday. If your "day pick" is like monday 17th, or later than 23rd, fewer crowds. If using online be SURE u r booking open-deck at least 1 way.
• I was there several days without a car & used bus (these days, they also rent e-bikes and I think ATMs... if u are conversant with them you can be OK, one main road only not that hilly, take care)
• About the bus - Plus...it gets you to the Temple, you spend a bit of time, wandering in/around, taking pix. Then what? Bus schedule very sparse, perhaps better in August. What I did was set off down the road about 100 yds, then on downill side saw sign on tree "Short-cut"..., scrambled down a path 200 yards or so and ... Presto! on the outskirts of beach village (Ag Marina??). I found a beach taverna, changed into swimsuit in its restroom, had a dip & sunbathe, sat at lunch table asked about bus schedule and bus stop, then had leisurely lunch, caught bus back to port.
• There are many port cafes for an adult beverage while waiting for ferry.... if you have a longer time, it's fun to brwose through the back-streets of port town, some fascinating small stores, not souvenirs but groceries and home-decor and hardware stores for the permanent residents, an initeresting look into regular greek lives.