Mirissa is in Southern Province.
Mirissa is a beautiful little village with a nice beach and two surfing points. One for advanced surfers, and one for intermediate ones.
Take bus heading to Matara from Fort in Colombo and ask to get out in Mirissa. A/C costs 370 Rupees. Regular costs 250 Rupees. Depending on when you go, a trip will take between 3.5 and 5 hours.
Alternatively, you can take the train from Colombo Fort to Weligama (7 km from Mirissa) and then go by tuk tuk to Mirissa. A 2nd class train ticket costs 220 Rupees. The train schedule is available here. Trips take between 2.5 and 3.5 hours (depending on the train).
Mirissa is a small village, you can basically get everywhere by feet. Walking.Cycling. Motorbiking are options. Available to rent everywhere.
Buddhist temple. Fisherman harbour. Beach.
Surfing. Bodyboarding. Whales watching. Swimming. Ayurvedic treatments.
Great roti in Dewmini Roti Shop more deep in the village.
In the beach bars around the beach. I recommend "Sunbeach restaurant" the staff sells alcohol and you can easily get grass there.
There is accommodation available for each type of traveller. You can find budget places starting from 1.000 Rupees in off season and 1.500 Rupees in season.