We go to Skiathos yearly,but it's quite small,so if your after history,or hiring a car and touring,then it's not really for you. The beaches are very good,and all over tan if you wish.
If your after similar temperatures to Cyprus,then wait until mid/late June. Heatwave the last week in June,Temps in 100's which,for me,was much too hot
Kefalonia is a beautiful island also,and perhaps worth hiring a car for a few days,although it's twlelve years since we went there