You want lovely beaches this is the place, don't be mislead these are the home of stingers and large estarine crocodiles (saltwater) to 4.5 metres. Try Dalywoy Bay, Cape Arnhem (2hr/60klm drive) and with permission Port Essington. North (60klms) is the Bromby Islands and the Wessels which is untouched and unbelievable!! Try for pics soon!!
Remember it's a long drive in through Katherine 600klms dirt passing through Aboriginal land (Arnhem Land). Or you can take the easy way and flyfrom either Cairns or Darwin and hire or barge a vehicle.