I think that’s a great plan. Santorini is a must and Milos and Naxos are two islands that have both great beaches and lots to do and see away from the beach. Renting a car and exploring the interior villages of Naxos is a must-do and so is a boat tour around Milos. If you’re willing to cut Athens to 2 days, I would recommend Santorini 5 days, Milos 3 days, Naxos 3 days, and Athens 2 days.
Located in the Caribbean Sea, this Dutch-owned island is perfect. Voted one of the best in the Caribbean, Curacao is like being in tropical Holland. The town is built in the Dutch style, but the surrounding areas are all tropical. Hit the beaches, lay in the sun, and party the night away. This may not be Gilligan’s Island, but what it lacks in privacy, it makes up for in romance and fun.
A complicating factor is that of origin-destination control ("O&D control"). Someone purchasing a ticket from Melbourne to Sydney (as an example) for A$200 is competing with someone else who wants to fly Melbourne to Los Angeles through Sydney on the same flight, and who is willing to pay A$1400. Should the airline prefer the $1400 passenger, or the $200 passenger plus a possible Sydney-Los Angeles passenger willing to pay $1300? Airlines have to make hundreds of thousands of similar pricing decisions daily.
Between Nydri and the next main tourism centre is the little port and beach of Mikros Gialos (small bay) that is a great little base for individual travellers for a day or three. The port village of Vasiliki on the southern underbelly of Lefkada is a haven for windsurfers: see this page or this one for more information. The little village is very pretty and is a good base for general holiday-making (as are the two other places). From Vasiliki there is a regular local ferry that runs to Fiskardo on Kefallonia (via Ithaca/Ithaki).
The United States, Australia, and to a lesser extent Brazil, Mexico, India, the United Kingdom, and Japan have "deregulated" their airlines. In the past, these governments dictated airfares, route networks, and other operational requirements for each airline. Since deregulation, airlines have been largely free to negotiate their own operating arrangements with different airports, enter and exit routes easily, and to levy airfares and supply flights according to market demand. The entry barriers for new airlines are lower in a deregulated market, and so the U.S. has seen hundreds of airlines start up (sometimes for only a brief operating period). This has produced far greater competition than before deregulation in most markets. The added competition, together with pricing freedom, means that new entrants often take market share with highly reduced rates that, to a limited degree, full service airlines must match. This is a major constraint on profitability for established carriers, which tend to have a higher cost base.
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).