Hi, I went to Sri Lanka in Dec 2018 and covered Pollonaruwa in North East, then went to Kandy in Centeral, Nuwara Eliya, Ela, Tissamaharam, Mirissa(south)and Colombo. I would suggest you stay atleast 2 days in Mirissa to fully enjoy the South coast. If you like, you can do Scuba Diving in Unwatuwa and Yala Jeep Safari. We also did Whale watching in Mirissa but do check the weather before booking your trip. In rough seas, you don't get to see any Whales or Dolphins and have to travel deeper, thus making you sea sick.
If you're heading to the west and south coasts and hill country, December to March is the best time to visit Sri Lanka. Sri Lanka’s tempertaure in March ranges from 24 degree celsius to 32 degree celsius. The country witnesses little rain later in the month which makes it even more pleasant to visit.