Hi, I would say try Vourvourou. Lovely beaches, shallow clear water and a nice little village. A bit out of the way, but you can always have a trip out to Marmaras one day to do touristy things (with a Greek twist, it is not a terribly touristy town).
If you want the main tourist area with lots to do, then Kassandra peninsula, and I would probably try for a hotel like Agean Melathron or Kassandra Palace which have nice beaches, but they are umbrellered !
Sithonia, the middle peninsula, is more sandy and isolated and has more camp sites.
From our experience, islands are not always that great on the beaches, only having one or two really sandy ones and the rest are shingle or rock. Having said that Chios is lovely, a working island and it does have a few sandy beaches about and not touristy at all.
Skopelos is nice and beautiful scenery, but from memory only one really sandy beach.
I think Corfu has lots of nice beaches, but we have not been there for 20 years and I think it is now a bit touristy?
Jane