Ko Samet's bar scene is concentrated on Hat Sai Kaew and Ao Phai, with fire shows on the beach most evenings during the high season (November to April). The beach bars are open-air, run their own sound systems, and stay open late — the island has a reputation as a party destination among Bangkok weekenders.
Naga Bar on Hat Sai Kaew is consistently busy and runs nightly fire shows with performers twirling flaming ropes and poi over the sand. The drinks are standard bucket-and-mixer fare but the setting is genuinely fun.
Ao Phai Bar (several places use this name) on Ao Phai is a quieter option — cocktails on low cushions facing the water, good enough to be a destination rather than just a stop.
For something lower key, the bars in Na Dan village serve cold beer at normal Thai prices in an atmosphere aimed at locals rather than tourists.