If you are looking for a nice, short day trip from Selcuk, you can head to Yoncakoy Beach.  It is a small beach, devoid of foreign tourists, and has 2 restaurants, one on either side of the beach.  They have beach chairs and umbrellas for rent and beverages to drink.  To access the beach you walk through a small community of vilas, most likey vacation homes.  It is only 12km from selcuk and you can catch a local bus from the bus station in selcuk.  The bus line has several other beaches listed that might be worth a stop.