The best way to travel in Sri Lanka is to travel to middle of the country and then come down to the coast on your way back home. This way you can relax on the beach on the way back home.
The best places to visit in Sri Lanka are Nuwara Eliya Tea plantations and Sigiriay rock castle.
If you want to quite area to stay on the beach Tangalle beach on the South Coast and Pasikuda Beach on the East coast is recommended. Mirissa is a nice place if you want to go out and party during the night.