Islands where to go

Hi All,
Myself and three friends are going to Athens for one week in early september, we planning on staying a couple days there and then going to an island for the rest. We are looking for a lively place (lots of pubs/clubs) with good beaches (no pebbles thanks) also a bit of sight seeing. I haven't got a clue where to go and would appreciate any advice on what island and what town/village would offer us all these if at all possible.