Panama is an underrated destination in Central America, including the San Blas Islands. This is a popular spot for sailing and boat tours, though there are also some resorts in case you’re looking for a more luxurious stay. Generally, the islands are quite rustic and make for a great off-the-grid island getaway. There are tons of beautiful spots for good sailing, diving, and snorkeling.
To sum up: a lot depends on your own stamina because island hopping means packing and unpacking, getting on and off buses and ferries. Limit your choice of islands to perhaps one or two less than you think you can manage. Maximise transport links to avoid backtracking or port-transferring and since you are traveling high season be aware that you will usually need bookings ahead at most places. It is possible to turn up on an island and not find a place to stay or have to make do with a third-rate option.
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.
Since St. John is part of the U.S. Virgin Islands, U.S. citizens can visit without a passport. There are quite a few bargains on both St. Thomas and St. John right now, so it’s a good choice for travelers on a budget. Snorkelers should not miss visiting Trunk Bay, one of the best snorkeling spots in all of the Caribbean for seeing amazing marine life.