My wife and I will be visiting Sri Lanka for a few weeks this November/December. I'd love to find a secluded beach to just relax on for a few days somewhere in the southern or west coasts. So far Mirissa seams like the best option but do you have other recommendations?
In the south, try the area near the Weerawila Airport. It's close to a 3 National Parks including Yala. It's also not too far from Kataragama. You can go up to Lunuganwehera Reservoir. There's a school you can volunteer to teach kids called Bridge to Peace and some very ancient Buddha Statues that have some mystery to them.
Of course near by Hambantota has grown a lot the last decade and is busy with tourism.
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If you are looking for a truly secluded beach I would head more towards Tangalle over Mirissa. In particular check on Google maps for Surya Garden Guesthouse , you don't need to stay at that hotel exactly, but within a few hundred yards in either direction will put you on a completely empty yet clean sandy beach. Also, you will find a fantastic little restaurant stationed in a tiny hut right in the center of the beach full of truly delicious food for super cheap. Of the ten or so beaches, we visited while we were in Sri Lanka this one was by far the quietest and cleanest.
We just got back from a lovely trip to Sri Lanka and stayed at Unawatuna. It's not exactly secluded but very close is a gem called Jungle Beach. we had the best time swimming there as there are no waves, just calm, deep, clear water. Arugam Bay I hear is amazing and is on our list of places to go to next.