Considering your interests (great food, hiking, beaches, nightlife unimportant) then Naxos should definitely be your other island. (And Naxos has many daily ferry connections with both Santorini and Athens.) Also, Athens needs at least one full day to explore so you should drop any thoughts about Delphi or Nafplio. Also, I would look into flights from Athens to Santorini on your night of arrival. If you could get to Santorini that night (and move your day in Athens to the end of your trip) you’d almost gain an entire day and could spend two nights on Naxos.
Transport between the three islands relies on local ferries and these are unsophisticated ‘landing-craft’ style boats that do little more than ferry passengers and vehicles in Spartan comfort, but they are very functional and vital to the inter-island communication. There is plenty of on the ground support excursions and infrastructure and the islands are well-used to tourism; the only exception is that travellers will need to use a bit of independence in getting between the islands.
Unlike its parched Cycladic neighbours, Andros is refreshingly lush. The ancient Greeks called it Hydroussa ('watery') because of its plentiful waterfalls and springs. Marble lions spout ice-cold mineral water at Sariza and Menites. Streams signposted by pink ribbons of oleander tumble down valleys criss-crossed with stone bridges and footpaths. (Some 170km of hiking trails have been waymarked; find out more at Andros Routes). Falcons and monk seals duck and dive at blissful, blustery beaches including Zorkos, Vitali and Vori.
Most airlines use differentiated pricing, a form of price discrimination, to sell air services at varying prices simultaneously to different segments. Factors influencing the price include the days remaining until departure, the booked load factor, the forecast of total demand by price point, competitive pricing in force, and variations by day of week of departure and by time of day. Carriers often accomplish this by dividing each cabin of the aircraft (first, business and economy) into a number of travel classes for pricing purposes.
In 2017, 4.1 billion passengers have been carried by airlines in 41.9 million commercial scheduled flights (an average payload of 98 passengers), for 7.75 trillion passenger kilometres (an average trip of 1890 km) over 45,091 airline routes served globally. In 2016, air transport generated $704.4 billion of revenue in 2016, employed 10.2 million workers, supported 65.5 million jobs and $2.7 trillion of economic activity: 3.6% of the global GDP.
The almighty church has kept nightlife in check. If you must see and be seen, head to quietly glamorous Astivi or Stoa bar, on miniature Agia Lesvias square, in Hora. Beach life is generally languid and low-key; Psili Ammos and Livadi Geranou are our favourite hideouts. Dinner reservations are essential at Benetos, for Med-Asian fusion on an organic farm, and Lambi for grilled fish on a purple pebble beach.