It’s best to visit Greek islands within the same group. For example, I wouldn’t recommend visiting Corfu and Santorini on the same trip as they’re on opposite sides of the country. Instead, visit islands in the same island group: the Cyclades, the Sporades, the Dodecanese, the Ionian, the Saronic, and the Northeastern Aegean. For one, they’re close to each other. And two, they have frequent ferry connections with each other. For first time visitors to Greece, the Cyclades make the most natural and convenient introduction.
Can you see all the major sights in Athens in one day? No. But you can see the Acropolis, Acropolis Museum, and the top historical sites of the Plaka in one day. If you had an extra half-day then visit the Archaeological Museum in Exarcheia. That still leaves many great sights but you will have seen all of the iconic Athens attractions. Adding Naxos is always a good idea. You might even enjoy it more than Mykonos (but no nightlife like Mykonos).
Ferries are not really cheap any more – certainly not like they used to be – but the vessel fleet is better, safer and faster than in previous years so the cost is justified. You can get exceptionally cheap deals on longer-haul routes if you are prepared to forego a booked seat: €14 v €40 on a run to a mid-distant island, but it’s probably wise to consider taking fast catamarans to get to core islands (Mykonos, Santorini, Paros etc.) and even then, go the extra 10/15% for Business or even VIP class for the extra comfort. On longer hauls it can be cheaper to fly if you seek out flights online and book beforehand.
In the 1990s, "open skies" agreements became more common. These agreements take many of these regulatory powers from state governments and open up international routes to further competition. Open skies agreements have met some criticism, particularly within the European Union, whose airlines would be at a comparative disadvantage with the United States' because of cabotage restrictions.
Moreover, the industry is structured so that airlines often act as tax collectors. Airline fuel is untaxed because of a series of treaties existing between countries. Ticket prices include a number of fees, taxes and surcharges beyond the control of airlines. Airlines are also responsible for enforcing government regulations. If airlines carry passengers without proper documentation on an international flight, they are responsible for returning them back to the original country.

I’m sorry if you covered this in another part of your site, (either I’m technically challenged or there just isn’t a search function for your site) but I’m curious about Corfu. From what I can tell, you mentioned it once in your site under your post about best beaches. The Paleokastritsa area is something that has caught my attention for a while and is on my list, and then I saw pictures of Nissakids Bay and that looked kind of amazing.
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.