5 days would be great, visit Bentota for 3 nights. Take a day trip to Galle. Later visit Colombo for 2 nights. If you can add a few more days then maybe you can visit Sigriya and Kandy.
Sri Lanka is a friendly and safe country . You Can go ahead and plan a solo trip ! You can visit Kandy , Nuwara Eliya and Colombo . 5 days are good enough Have a great trip