My boyfriend and I will be traveling to Greece on August 1-11th. We have 10 days. Is this a feasible itinerary for a couple in their early 30s who want beach, relaxation, good food, boating, and some history? Fly into Athens have one full day there then fly to Naxos for a day and a half, Milos for 3 nights, then Santorini for 3 nights, then back to Athens for our flight? We chose Milos over Naxos at first, but after reading your blog it seems the beaches in Naxos may be better?
Analysis of the 1992–1996 period shows that every player in the air transport chain is far more profitable than the airlines, who collect and pass through fees and revenues to them from ticket sales. While airlines as a whole earned 6% return on capital employed (2–3.5% less than the cost of capital), airports earned 10%, catering companies 10–13%, handling companies 11–14%, aircraft lessors 15%, aircraft manufacturers 16%, and global distribution companies more than 30%. (Source: Spinetta, 2000, quoted in Doganis, 2002)

We’re planning a Greece trip to celebrate our 25th anniversary in September 2017 We plan to arrive in Athens and directly take train to Kalambaka/Meteora (2 nights), then down to Delphi (hotel stay in Athens), a day to see sights in Athens, a day trip to Nafplio and then leave to explore islands. Our plan is to go to Naxos for 4-5 nights, then to Santorini (for our anniversary) for 4 nights, and then to Crete for 5-6 nights. I would like to see Delos and wonder if it’s possible to stop in Mykonos, do the tour to Delos and still be able to catch ferry to Naxos the same day?
Comfortable accommodation in May for 2 persons can be found for between €40 and €80 per night. A meal for two that includes a starter, two main meals, salad and a litre carafe of local wine will cost you around €25-35. This can vary widely depending of level of establishment you eat at. A cheap vegetarian dish (pulses or vegetable) will set you back by no more than €5-6 a plate. If you get your breakfast included at the hotel, that is good because breakfast can add another €15 for the two of you per day. 

Operating costs for US major airlines are primarily aircraft operating expense including jet fuel, aircraft maintenance, depreciation and aircrew for 44%, servicing expense for 29% (traffic 11%, passenger 11% and aircraft 7%), 14% for reservations and sales and 13% for overheads (administration 6% and advertising 2%). An average US major Boeing 757-200 flies 1,252 mi (2,015 km) stages 11.3 block hours per day and costs $2,550 per block hour : $923 of ownership, $590 of maintenance, $548 of fuel and $489 of crew; or $13.34 per 186 seats per block hour. For a Boeing 737-500, a low-cost carrier like Southwest have lower operating costs at $1,526 than a full service one like United at $2,974, and higher productivity with 399,746 ASM per day against 264,284, resulting in a unit cost of 0.38 $cts/ASM against 1.13 $cts/ASM.[71]
The earliest fixed wing airline in Europe was Aircraft Transport and Travel, formed by George Holt Thomas in 1916; via a series of takeovers and mergers, this company is an ancestor of modern-day British Airways. Using a fleet of former military Airco DH.4A biplanes that had been modified to carry two passengers in the fuselage, it operated relief flights between Folkestone and Ghent. On 15 July 1919, the company flew a proving flight across the English Channel, despite a lack of support from the British government. Flown by Lt. H Shaw in an Airco DH.9 between RAF Hendon and Paris – Le Bourget Airport, the flight took 2 hours and 30 minutes at £21 per passenger.
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.
Major airlines dominated their routes through aggressive pricing and additional capacity offerings, often swamping new start-ups. In the place of high barriers to entry imposed by regulation, the major airlines implemented an equally high barrier called loss leader pricing.[38] In this strategy an already established and dominant airline stomps out its competition by lowering airfares on specific routes, below the cost of operating on it, choking out any chance a start-up airline may have. The industry side effect is an overall drop in revenue and service quality.[39] Since deregulation in 1978 the average domestic ticket price has dropped by 40%.[40] So has airline employee pay. By incurring massive losses, the airlines of the USA now rely upon a scourge of cyclical Chapter 11 bankruptcy proceedings to continue doing business.[41] America West Airlines (which has since merged with US Airways) remained a significant survivor from this new entrant era, as dozens, even hundreds, have gone under.