whilst on Samui, if you catch the full moon party (once a month party) on Koh Phangang its a must. never too young, its certainly an experience, we managed two in early mid naughties.
if you enjoy your party enhancers though dont be tempted to buy at party, more likely undercover and dont wanna be nicked out there.

if you like your spicy food you gotta have the green papaya salad sold by the hawkers on the beach for maube 60 to 100 bhat.

have fun