Islands the first week of October

Why not skip all 3 of them, and have a different experience entirely? How about coming back to Pireaus and jumping on one of the (almost hourly) ferries to Aegina?? Here's a wonderful step-by-step photo album of a June daytrip to Aegina that shows many of its highlights -- http://www.flickr.com/photos/36264706@N03/sets/72157621604646139/detail/ -- in summer it's best to go there M-F because Athenians flock there on weekends, but in late September I don't think it's a worry. And Aegina DOES have the best-preserved ancient temple of Any Greek Isle, on a high hill with 360° view of saronic bay. i If you want a final beach fling, you could stay at a place like this: http://www.aeginagreece.com/aegina/pages/beaches/la_palma_marathonas_cafe_bar_restaurant.html -- it's been well-reviewed (in Greek); a friendly cafe that has a side bldg w. rooms/apts.

If you wanted to split it up, you could even do 2 days there, and then take a 10-minute motorboat ride to the tiny adjoining isle of Angistri ... its so small that you bicycle to the other side. this place - http://www.rosyslittlevillage.com - has good reviews.

I have nothing against Andros except that my shelf of guidebooks seems to struggle to find something interesting about it... some people mention hiking, but Naxos is waaay ahead in that dept. Tinos seems to require a car to get around to its sights, which apparently consist of dovecotes and hand-crafted lace-making. I have a friend who used to "commute" from Tinos to work in Mykoknos, and liked it because it was so quiet... and that was in summer! Syros is worth a night's stay just for its remarkable town square in Ermopoulis, but in a 3-day stay I found it to be 99% Greek visitors, and not that welcoming to English-speakers, my first such experience in visiting 25+ islands. Probably because it's so prosperous it doesn't need nongreek visitors.

Advantage of Aegina/Agistri is that when you're ready to head for Nafplio, you can jump on a 7 or 8 am ferry with people who commute daily (it takes 45-60 minutes max) and be on the road to Nafplio by 9 or so.

I agree that Aegina would be a wonderful choice since it's so close to the mainland and has a nice main town with waterfront and most everything stays open. Archeological sites, museums, beaches, seaside villages, walking, hiking, ancient olive grove and abandoned Byzantine Village in the Hills.

I have been to Tinos and Andros in early October with Tinos turning out to be a gem of a Greek Island . . . far better than I expected with some of the most beautiful villages I've seen in Greece, Spectacular mountain ride in the west side of the island, Dove Cotes and magnificent terracing all over the island.

Since October is a very quiet time on most Greek Islands I stayed in Tinos Town, the largest town on the island with everything up and running and lots of shops, markets and tavernas. The rest of the villages were quiet but filled with Greek Life rather than tourists everywhere. I think you'll find Tinos worth a visit in early October.

Andros was a bit disappointing and Syros would have been better to combine with Tinos.

Still Aegina would be a good choice but don't overlook Tinos which is easy to get to by ferry.

You should check various ferry websites for schedule such as:

https://www.ferryhopper.com/ (just enter your information to see whats running)

or https://www.gtp.gr/ferryconnections# (click on the + sign to zoom in then pick an island to see what the ferry runs are.)

We enjoyed Syros, the administrative capital of the Cyclades. Few foreign tourists.
However, if you are having difficulty getting there and back, I would add those days to beautiful Napflion. In addition to visiting the many antiquities that surround it, we also spent a day on the pretty green island of Spetses. We drove to a dock where there was a water taxi. There also were Day Tours offered to Hydra and Spetses from Napflion.

Thanks for all the input: Another question
Should we be purchasing ferry tickets in advance for Santorini to Naxos on Sept 24 or Naxos to Mykonos on Sept 28?
Again greatly appreciated

You shouldn't have to pre-buy ferry tickets for that time of year as it's low- or off-season time. The ferrys that go to those islands are huge and can carry hundreds of passengers so you should be fine.

But if peace of mind is needed then you can pre-order tickets . . . but I think all you need to do is show up at the ticket office and buy your tickets. Just make sure you get there about 1 hour ahead of time. There is no check-in or security to go through . . . buy your ticket, walk to the ferry, go up the "gangplank", put any luggage in the below deck racks (perfectly safe) and walk up stairs to your seats.

A lot less stress than at airports and no long waits.

tommy is perfectly correct about how to buy tickets directly from the ferry office ... But in my 12 visits to greece I have not bothered with either alternative ... I have not wanted to bother about doing something online worrying about entering info correctly, credit card etc.and then getting a voucher that I STILL have to go to the ferry office on the pier to exchange for a ticket ... OR getting there early, finding the office etc. I usually stop at a ticket agency -- there is one right at the airport -- or one by my hotel... in central athens, there are agencies in almost every block. I go to a counter, tell them what I want, give them cash or credit card & walk out with my ticket. I think they charge like €1 convenience fee, but hey. Then I can show up at the ferry any time before it sails, depending on what type of ferry it is. When island-hopping, I get to an island where I'm going to be for several days, and buy the next ticket there, a few days ahead. Granted, I go offseason, May-June or September, and Not on a holiday weekend If i were travelling July-August, I'd have to plan farther ahead, maybe even online.

NOTE: When you buy online, you don't have a choice of location of your seats within a certain ticket class -- reserved economy, business class etc. In a ticket office you can ask to see a layout of reserved seats, and request seating by a window etc.

NOTE #2: On a catamaran (passenger-only or passenger + vehicle, a larger type), there is NO outside seating, all seats are inside and reserved. So, these are the only choices, and you need not get to a ferry early in order to maximize your seat choice.
BUT -- If you take a "conventional" ferry -- BLUE STAR, etc -- usually passenger capacity between 1000 - 1700) there are more choices. There is outside seating on 2 upper rear decks. If you buy a plain-vanilla "economy" ticket, you are entitled to deck seats on a first-come-first-served basis... or you may sit in a cafe indoors, or in seats lining a corridor. For about €4-5 more, you can buy a "reserved economy" ticket --sometimes called "aircraft seating". These are numbere reserved tickets, comfortable upholstered leanback seating with LOTS of legroom, tray tables, footrest (nothing like the torture-seats of today' airlines!!). "Reserved economy" ticketholders are ALSO perfectly free to sit outdoors in whatever seats they can find ... and if it's a long voyage (more than 2 hrs) & they get tired of sun and sea, and want to go inside to read or nap, they have a reserved seat waiting.
In the off-season, like May or October, during midweek these large ferries are half-full ... and if you want to save €5, you could take a chance on plain "economy" and then if you go inside just look for an empty seat, and be a "silent squatter". If taking a ferry outbound from Pireus on a FRiday, or inbound on a Sunday, June - Sept, it's prudent to make sure of a seat.

In any case, if you want to make sure of a deck seat, do get to the pier early to be near the head of the passenger line. Drop your big baggage promptly on the vehicle deck (there are racks), and scamper up the escalator or steps to snag a good deck view.