For a week on the Amalfi Coast, if it is truly 7 nights, I would split my time between Sorrento and Ravello. You can use Sorrento as abase to go to Ischia or Capri for a day, revisit Pompeii or go to Ercolano, spend a day in Naples. Ravello is high on the cliff overlooking the Mediterranean and is very relaxing. You can go to Paestum, Positano, Amalfi, Atrani, and many other tiny towns from here. Ravello itself has two magnificent villas to see, good restaurants.
I would not add Siena to this itinerary. Personally I would find it a hassle to take that trip. You'd have to get to Sorrento, then Naples where you catch a FrecciaRossa to Florence. From Florence you change to a bus or another regionle train to Siena. But it depends on your travel priorities: the outdoors and natural beauty of the AC, or the museums and churches of Siena.