A night market sets up in the town centre (away from the beach) most evenings, with a concentration of food stalls that cater to the town's Thai residents rather than weekend visitors. This is where you'll find som tam, grilled meats on sticks, pad krapao, and the sticky mango rice desserts that Thailand does better than almost anyone else. Everything costs roughly half what the same dish would at a beachfront restaurant.
The market is at its best on weekend evenings when it expands and picks up pace, but even on a quiet weeknight there are enough stalls to make an interesting wander and a cheap dinner. The location shifts slightly by season, so ask your guesthouse for the current spot.