the ferrry ride from Athens/Piraeus to Santorini/Thira

You can check ferry schedules and pricing here: http://www.gtp.gr I'm not sure if September's schedule is complete yet so keep checking for updates.

Hellenic Seaways offers the highspeed ferry on that route so you can check their website as well.

Have you thought of flying? Two years ago we flew into Athens and took an Olympic Airlines flight to Santorini with just a couple of hours spent in the airport. From Santorini we took the ferry to Naxos and and after 5 days ferried back to Pireaus. After a long flight from SF to Athens we just wanted to arrive in Santorini as soon as possible. As I recall the price was reasonable when comparing to the ferry price and number of hours of travel. If the airport is a convenient option you might check the plane fares as they often go on sale months in advance of the flight date.

I second looking into flying. The fastest ferry is about 5 hours by fast catamaran, though if the weather turns, they can be rough. The slower more stable boats are 8 hours. Both are a bit long on a ferry for me.

Blue Star runs a pretty comfortable, large "conventional ferry" that is planned to depart Pireus at 7:25 and get to Santorini at 14:15, but ferry schedules are always subject to influence by weather and other factors. www.greekferries.gr is a booking service that can connect you to several ferry companies.

No over-65 discounts on ferries, planes, or many of the sight-seeing venues. Makes sense -- we retirees are a huge part of the Greek tourism picture, they don't need to lure us with discounts!