You’d have to check the schedule for your specific dates but I think Santorini then Milos then Paros then Naxos and Athens would probably be the best order. 3 days in each sounds great. Or a small tweak: 2 days in Milos and then 4 days in Santorini (splitting time between 2 of the 4 caldera towns) or even 4 days in Paros (splitting time between Naousa and Parikia).
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.