Tsambika is lovely, long and sandy. The beach at Faliraki is also good. I love a little beach called Kavorakia which doesn't get crowded.Further afield Gennadi is a great beach, I could go on and on!

Agree Tsambika is HUGE and a great beach (strong undercurrent in some spots so don't swim out too far). There's a fabulous little beach just south of Lardos (i forget the name but it's the first beach after Lardos). Ladiko or Anthony Quinn Bay near Faliraki are really pretty little bays.
And on the west coast Kamiros is well worth a visit. Like Anne i can't just pick the one - there's so many i love!!

Mustn't forget St.Agathi and Stegna etc. etc. etc.!!

Kalithea Bay (just north of Faliraki) is superb for snorkelling. A rocky beach with beautifully apoointed sunbeds and umbrellas. The water is fantastic. For sunbathing? St. Paul's at Lindos is very contemporary with wooden beautiful wooden lounges and decking areas for a relaxing bar. The bay itself is clear and wonderful for swimming, the beach is sandy. Any of the beaches on the Mediterranean side are lovely and sandy - on the Agean side shingly and windy, but both have there plus points. It depends on what you are looking for in a beach! You will enjoy at least one of them!