Sri Lanka’s compact size and the accessibility of most major attractions means that even a week will allow you to visit a number of different areas. From experience and you gain maximum enjoyment form the island, at least two-week stay is advisable.
Note: In two weeks you either go round the island South (mostly beeches) or North (cultural) make sure you leave time between destinations ie. to get from Colombo to Kandy by car can take up to 8 hours depending on your route and how you travel, in my view travel by bus or train is always the better option:
Colombo - Galle Fort - Unawatuna - Dalawela - Thalpe - Koggala - Midigama - Weligama - Mirissa - Matara - Dondra - Dickwella - Tangalla - Hambantota - Bundala National Park - Tissamaharama - Yala National Park - Kataragama.
Kandy - Kalpitiya - Wilpattu National Park - Anuradhapura - Nallur Kandaswamy temple - Nagadeepa Purana Raja Maha Viharaya - Kadurugoda Temple - Shree Nagapooshani Amman Hindu Temple - Miracle Springs of Keerimalai - Bottomless Well of Nilavarai - Casuarina Beach - Jaffna Fort - Fort Hammenhiel