1. Go for an early morning swim
2. Walk to the harbour and watch some early morning fishermen activity
3. Eat a roti or some kottu at 'The No.1 Dewmini Roti Shop'
4. Have a tour of the Hundungoda Tea Estate
5. Walk along the beach in the evening and then find a spot, at one of the many restaurants, and watch the sunset with a happy hour drink in hand

At first glance Mirissa seems like the perfect example of tourism gone horribly wrong.  It was exactly my feelings towards the place.  But I was wrong.  Just wait, be patient, there's a good chance it will grow on you.  It's an acquired taste; a bit like jackfruit.