hi harshp.
it seems like a lot of travelling (distance) in a relatively short amount of time. kalampaka to athens via santorini - that's just over 1000km.
if you have your heart absolutely set on santorini, then by all means, you should go there. it is a beautiful island and i would go back in a heartbeat. but it is a long way and personally, i would maybe think about one of the islands closer. it takes between 5-8 hours (each way) to get there from athens on a ferry, and in my opinion, that is just way too long. you can get a plane, but the cost may be prohibitive. also accommodation at the last minute in oia... i really don't like your chances. but if you are willing and can find somewhere, then make it work and enjoy every minute of it!!! i stayed in amoudi bay (amoudi villas) which is under oia, so many stairs!!!! but more than worth it. make sure you get down and try some of the tavernas down there. and santo wines is not to be missed. also make a point of doing a semi-private captain ted cruise!
just to give you another option, i recently spent time in the saronic islands of hydra, poros and aegina, and they are simply stunning. you can do a day trip to all three from athens, but i would definitely look at staying on the islands for a few days themselves. they are much closer (around 1-2 hours from piraeus), and you will have more time to explore them instead of sitting on a ferry for 10-16 hours!
hydra is stunning. it is rather quiet and there are no cars. it is a great little island to simply relax and soak up the sun and greek hospitality. imho, it was a smaller and quainter version of santorini (and less tourists!). one of my favourite places in the world.
aegina was a great island, i enjoyed every minute of time spent there. we stayed near vathi, and had beautiful beaches to ourselves, some amazing food, and we did a lovely hike up to the temple of aphaia, which was breathtaking. don't forget to try the pistachios!!!!
poros itself wasn't the most touristy island, there are no big attractions, but it is still beautiful island to explore. we stayed with friends on poros and took the car ferry to the mainland and drove up to see the stunning epidavros, which was a highlight of the trip for us. there is a new road so it was an easy and pleasant drive along the coast and through the mountains. i would love to spend more time out on the peloponnese.
if you can hire a car and drive all the way down from athens (apparently 3 hours), and give yourselves time explore along the way, then you can get a ferry across to poros, or even hydra from slightly further down. public transport didn't seem like a viable option in this regard, so sorry if driving isn't something you can do.
i agree with the previous poster - leave your athens days til last, just in case :) also, i booked all of our ferry tickets for a trip mykonos-santorini-kos-bodrum all in advance online, and i stressed the whole time about being able to pick them up and find the offices, sometimes they were closed and it was just not worth it. i decided it would have been easier to just go down to the port the day before and purchase a ticket then. last year i traveled between the saronics and piraeus in august and had no trouble getting ferry tickets on the day - obviously we were there an hour or two before departure. for busier routes, i would purchase the day before, just in case.
there is a great website i use called rome2rio.com, and usually they have a rideshare option if there is one on the route. can't help more than that sorry :)
i just wanted to let you know that there are other options, not just the stunning santorini. i've been to 11 greek islands, including santorini, and they are all different in their own way, but all brilliant and worthy of a visit. whatever you choose to do, enjoy yourselves. greece has a special place in my heart and everyone should get to experience the greek hospitality and sun at least once in their lives :)
