The Mon Bridge is Sangkhla Buri's signature sight: a long, rough wooden bridge crossing an arm of Khao Laem Reservoir to Wang Kha village. It is photogenic, but it is also useful, carrying villagers, monks, schoolchildren, and visitors across the water all day.
Go at sunrise if you can. The light is soft, the lake is usually calm, and the town feels much further from Bangkok than the map suggests.