Fethiye / Hisaronu / Oludeniz / Kabak area in southwest Turkey is great. 12 islands tour is nice. And St Nicholas Island.
http://beta.trekearth.com/viewphotos.php?l=7&p=1466650
I have swum there in May but unless it's sunny / no wind the water may be a little chilly in April. Most of the offshore islands near Turkey are Greek. Rhodes, Kos, Samos, Chios, Lesvos, etc. Istanbul can take 5-6 days. I still haven't seen the underground basilica or done a Bosporus cruise! You can get a direct Pegasus domestic flight from SAW airport in Istanbul to Dalaman Airport then a shuttle bus from there to Fethiye. Usually there is a bus to meet the flight. Avoid an expensive taxi.
Bodrum and Marmaris are the other 2 Mugla province resorts, but I and other regulars on the Turkey LP forum prefer Fethiye area. Because I lived in Turkey for a year the restaurants in Hisaronu were great. Chinese, Italian, Mexican, Indian, English breakfast etc. I was tired of kebabs and salty Turkish cheese and olives so it was a real treat. The town of Oludeniz is ordinary but the beach and lagoon are fantastic.
http://beta.trekearth.com/members/ChrisJ/photos/Middle_East/Turkey/Aegean/Mugla/
You could get a bus from Fethiye to Pamukkale Travertine terraces and visit Cleopatra's pool in adjacent Hieropolis where the thermal water is always 37c. Then to Selchuk/ Ephesus and Kusadasiport for Samos > Naxos / Paros or Delos > Santorini (stay in Oia or another area) > Milos > Athens/Delphi.
http://beta.trekearth.com/viewphotos.php?l=5&p=1486520
http://beta.trekearth.com/viewphotos.php?l=7&p=1432746
http://planetden.com/wp-content/uploads/2014/04/AJP_1539.jpg
I am in Kosovo at the moment but travelled through Serbia, Bosnia, Croatia, Montenegro and Albania as well in the past 2 months. Split was quite busy, and the harbour was smelly, but Vrboska on Hvar Island was chilled out. I enjoyed the cold wine in the bar atop the fort in Hvar town too! Korcula was nice. Some Croatia snaps:
http://beta.trekearth.com/members/ChrisJ/photos/Europe/Croatia/