If you're making a day of it, head over to Arran or Bute. Local buses will take you to the beachy bits, i.e. Ettrick Bay on Bute; Arran you've got Brodick, Lamlash, Whiting Bay etc. Nice wee beaches around Cumbrae also. Will all be quieter, cleaner and prettier than Troon, Ayr, Largs etc. Plus you get to go on a boat!