Sounds lovely. From what I can find about Nissaki its quite a small village with a few Tavernas and shops. The beach looks tiny but as we're looking at a Villa we probably won't use the beach much. Looking at mid may as I don't like it too hot.
We went to kalami a bit further north on the same part of the coast in early july and it was gorgeous. Very civilised atmosphere, friendly locals, a few tavernas and bars but generally quiet and unspoilt, with nice clean beaches. I would go again if I could this year! From what I recall nissaki was a bit busier, but still nice.