Sifnos is a small island and has not many things to do, except good nightlife and relaxing beaches. I think three or four days there are enough. Better stay a few days on Sifnos and island hop on Serifos or Kythnos. Faros and Apollonia are really Cycladic villages and have nice, reasonably priced hotels. Platis Gialos and Chrissopigi are the best beaches and some beach bars are found around. Transportation on Sifnos is horrible, so rent a scooter to get around more comfortably.