The thing to do in Surin is to chill out at the beach, which is nicely separated from the hotels and the main road by a disused golf course turned into a park. The beach is lined from end to end with restaurants and cafes, which will happily rent you a sun lounger and umbrella for around 100 baht/day.
Surfing: Surfing here can be good, but all of Phuket is not consistent for surfing waves. Something to do in between relaxing. There are surfboards and bodyboards to rent on the beach or from the Saltwater Dreaming Surf Shop back on the main road.
Snorkeling: You can snorkel here in the dry season from October to April. around the headland at the northern end and out to the headland at Pansea Beach.