Muhudu Maha Vihara is an ancient Buddhist temple complex at Pottuvil, a few kilometres north of Arugam Bay, sitting right on the beach where the sea meets an ancient dagoba. It's one of the oldest Buddhist sites in Sri Lanka's eastern province — some ruins date back over 2,000 years.
The site is small but atmospheric. The main dagoba has been partially restored, and the surrounding ruins include carved stone pillars and image houses. Very few tourists make it here despite its proximity to the bay, which means you'll usually have the place to yourself except for resident monks and the occasional local pilgrim. The setting — ancient stonework with the Indian Ocean right behind it — is unlike anywhere else in Sri Lanka.