athens is interesting if you like shopping and ruins.
anyway, 1 day to see acropolis is good enough, and plaka, monastriki flea market.
all within walking distance of this hotel : Pan Hotel
(abt 90+Euros/nite, simple b'fast incl, i love the toast and meusli). http://www.panhotel.gr/en/main.htm
It's located just 2 mins from Syntagma square and the Parliment.
Possible to take airport bus x95 to syntagma square and walk some 100m to the hotel.
Airport bus is cheaper than metro, and takes abt the same time.
For the bus, you need to lug the baggage on the bus and off, but no need to climb stairs. For the metro at Syntagma square, no escalators to ground level, so a short fligth of stairs.
If you prefer a more up market hotel, the Grande Bertrange (spelling a bit wrong) is the grandest hotel in athens. Also at syntagma square (which is considered central). Worth popping in just to see the opulent lobby and washrooms.
To find good eats in Athens, find the 'best of athens' in the national library, a small compact book, good street maps and great food places (and shopping) marked out. Very light and easy to carry around.
In santorini, if you spend 3 nights,
you can take a one full day island cruise (go to a live volcano, thirassia and watch sunset at Oia, quite a good tour)---we booked ours with kamari tours, but i think other agencies were better.
The other days you can explore the island. One fantastic place to catch the famous sun set (apart from Oia) is the Santos winery.
Get a wine sampler set and sip wine and enjoy the sunset from there. No crowds like in Oia, and in my opinion, a more spectacular view.
In Oia, you can look for this "Seagull" Cafe. It is directly above the typical postcard view of santorini (check out my multiply), the one with the caldera view in the distance and the 2 blue roof with whitewashed walls. Here's a link to explore:
At either Thirassia or Fira or Oia, do take a donkey ride up. It's great fun!!! ;D
They only climb halfway tho, you do the rest, but very thrilling when they start to 'race' each other up. Keep your knees close to the donkey as you may scrape them on the walls/bushes.
Hope this helps. If you need more info, just holler.