Chaweng Beach road is probably the main place to go shopping at night on Samui Island. Other than a collection of stores, countless street stalls offers tourists nearly everything from apparels, sourvenirs to jewelry.
You can start at virtually every point on Chaweng from anytime after 6pm when the stall vendors set up shop. Goods are displayed clearly. However, bargaining is perhaps necessary as mark-ups in prices are sometimes several fold. Bargaining is unique to shopping in Thailand and can be a fun experience when you become good at it.
Waling the strip at night is also fun to do even if you are not interested to buy anything.
And if you forgot any item on your such as swim wear or diving masks, you can be certain that you can purchase them on Chaweng.