After you arrive at the Bandaranaike International Airport in Katunayake, you would probably head to the beach town of Negombo, which is about 7km away.
Negombo has one of the widest stretches of sandy beach in Sri Lanka. The beach here is ideal for sunbathing, but not so much for swimming. It can get rather crowded with locals, especially on the weekend. You may be able to join a game of volleyball or rugby played regularly on the beach, should you wish to get some exercise. There are plenty of cafes and restaurants along the beach that serve international cuisine. Most of the resorts in Negombo are located along the beach as well, providing a wonderful view of the ocean as you wake up to your first morning in Sri Lanka.