The large conventional car ferry between Paros and Santorini (www.bluestarferries.com) is VERY reliable and VERY stable. The seas are calm in May so that won't be any sort of problem. The flights are not a problem either, and you can sign up for the email newsletters of Aegean Airlines and Olympic Air to receive notice when they release their saver fares for May, sometime after the first of the year. They go for as little as €39, all taxes included, so per person you'd actually only be spending about €5 more than an economy ferry ticket back to Athens and it only takes 30 minutes as opposed to 8 hours. Since you're arriving early in the morning it might make better sense to fly directly to Santorini then take the ferry to Paros. From Paros you can either take a ferry back to Athens or fly there on Olympic Air for the mainland leg of your holiday. Saving the mainland for the end is also practical from the "Murphy's Law" point of view. You will have all those extra days to get back to Athens in case of some unforseen delay.

On Santorini take a look at Villa Marina. It's large enough for all of you and is in the most beautiful village on the island. There is only one flight of steps to reach the front door. http://www.santorini.com/hotels/villamarina

Follow brotherlee's suggestion and fly directly to Santorini, then ferry to Paros and from there to Athens.
Save 3-4 days for Athens where you can do the day trips that you mentioned. Not everything is doable though....
Sounion you can do on an afternoon, the sunset from the Temple is gorgeous, on your way back you can stop in Glyfada for dinner, an excellent southern suburb on the sea with many restaurants, cafes etc
Delphi is 2 hours away from Athens and can be visited in a day.
Nafplio is also 2 - 2,5 hours away from Athens and can be combined with Epidavros in a day trip.
If you stay 3 nights in Santorini and 7 nights in Paros you will only have 3 full days left for the Athens portion, unless you take an early morning flight from Paros.
Seeing Athens will take more than 1 day ( Acropolis and the New Acropolis Museum, Plaka, the National Archeological Museum, Lycabettus Hill etc) so you may have to skip a day trip.
Eleusis and Corinth would be the ones i'd skip,if i had to choose.
As for an apartment in Athens check out : http://www.vrbo.com
Once you have found something you like, check back with the forum, as not all apartments have a convenient location, even though they are in the centre.

As I understand it, your itinerary is constrained by the week's timeshare in Drios, giving you only two days between its end on Saturday the 26th and your flight home on Monday the 28th. That leaves you time only to return to Athens at the end of the trip. With that in mind, your idea of the Nafplion region first, then a flight or ferry to Santorini sounds a good one to me. From Santorini it is three hours by ferry to Paros on the new Blue Star Delos.

While on Paros, a day trip to Antiparos is very easy, and highly recommended. Day trips from Paros to Sifnos are impossible, as there are only two ferries per week between the two islands.

Be aware that Drios is on the opposite side of Paros from Parikia, the island's main port. It will be very quiet there in May, and bus service to other parts of the island limited, so you will need a car to get around. There are some nice beaches in the area, but not much else to keep two teenagers entertained. There is a well known beach bar at nearby Pounda Beach, but even that will be fairly quiet in May. Parikia, where most things are happening, is 45 minutes by car from Drios, and Naoussa, the next largest settlement, about 30 minutes away.

You may not want to hear this, but if it were me, I would ditch the timeshare to give myself more flexibility, and stay in Parikia or Naoussa. Accommodation is inexpensive on Paros in May.

Thanks everyone for your advice. Brotherleelove, thank you for the recommendation on Santorini - Villa Marina looks perfect for us! As well, the contact person also has an apartment in Athens that seems good too. It is located behind the Hilton Hotel - on Neofronos 8th. Can anyone tell us if this would be a good location in Athens?

Neofronos is located next to Hilton and to Vassilis Sofias Avenue.The location is very central
Nearest metro station is Evangelismos, 5-7 minutes walk from the Apartment.
There are restaurants and cafes nearby, in 10-15 min walking you can reach Syntagme Square ( 1 Stop from Evangelismos if you take the metro- blue line)
Coming from the airport all you need is to take the metro ( Blue Line) and get off at Evangelismos station, it takes 40-45 minutes.
Using the same line you can reach the port of Piraeus, with one change at Monastiraki station.
From Neofronos you can easily reach Ymitou Street with it's many restaurants and cafes, a very popular part of Athens with no tourists at all

Thank you all for your responses. We have now booked an apartment in Athens for our first three nights. It is located on Effroniou, near Merkouri. From what I can see, this seems to be near Neofronos so similarly well-located according to clausar's comments above. We have also booked Villa Marina for the following two nights and will keep our timeshare on Paros for the following 7 nights.

I am now trying to book our travel from Paros back to Athens. I don't think the ferry schedules are out yet, but I have read something worrisome about the 2011 schedule that indicates that there wasn't a ferry on Saturdays in low season (May). Is this true? Also, when I search for flights on Olympic's website, it shows no flights that day. I am hoping this just means that the schedule hasn't been released yet too. Finally, when I try to sign up for the Olympic newsletter, it switches from English to Greek which, unfortunatley, I don't read. I seem to only be able to get the home page in English. Any advice you have would be most appreciated!

Assuming we can get back to Athens on the Saturday, I was wondering if it makes sense to rent a car immediately (either from the ferry terminal or the airport) and drive to Delphi, and spend the night in the Delphi area so that we can tour Delphi on our last day. We could then return to Athens that evening to sleep before catching our flight the next morning.

Thanks again for all your help!

The Blue Star ferry operates every day. And May is not considered low season, so no worries there. Most likely you're right, all flights for May haven't been posted yet.

Best to book the car at the airport if you get a flight so you don't have to drive through Athens or the surrounding suburbs to get to the National Highway. That said, you may not have enough time to do this depending on when you arrive back in Athens on that Saturday. Delphi is several hours outside the city.

Effroniou Street is very central as well, not far away from Neofronos, but perhaps also not as quiet as Neofronos.
Being there take notice of 2 very good places to eat:
Karavitis : one of the oldest taverns of Athens ( Arktinou Str. 33)
Magemenos Avlos : a great popular restaurant.( Zalefkou Str 1- Proskopon Square)
Both offer great greek dishes, and are not touristy at all...

The idea of renting a car at the Athens Aiport is better than a pick up from the port... it is easy to drive to Delphi, it only takes 2,5 hours. On you way you can visit the Monastery of Osios Loukas, make a stop in Arachova ( posh village, very alive in the winter due to the proximity to the skiing center of Parnassos mountain) and overnight in Delphi.
next day you can see Delphi in the morning and then drive to nearby Galaxidi, a beautiful small neoclassical town on the sea. Then drive back to and overnight in Athens.

Keep checking the Olympic Air website, and summer season flights should be posted soon. Once released, tickets go quickly, as due to takeoff weight restrictions the Dash 8-100 aircraft can take only around 20 passengers on the flight from Paros back to Athens. There are usually three flights per day from Paros to Athens, and if you can get a morning flight it would give you all day to drive up to Delphi.

Blue Star ferries run year-round, every day including Saturdays, and in May there will be one leaving in late morning and another in early evening. In addition, there will a Hellenic Seaways highspeed sailing in May. Blue Star normally arrives in Piraeus at 3 pm and 11:30 pm, but not sure about the highspeed, as it varies from year to year. It is not necessary to prebook these ferries - just buy your tickets when you get to Greece.