I was there for six days.
I didnt enjoy Koh Samui much even in low season, tourists outnumber locals, and it feels a bit fake, like Disneyland no viable industry remains on the island that does not rely on tourism. The sea was too warm to swim in, too! Id recommend trying a quieter island, but if you end up on Samui anyway, heres my advice:
There are many restaurants right on the beach, none of which seemed better than the others. Go for seafood (you can choose your own). Everything was delicious, if in small portions. I had rubbery calamari/squid several times, so maybe go for something else. Remember, in Thailand prawns are large and shrimp are tiny, not the other way round as it is in the States.
SITCA (Samui Institute of Thai Culinary Arts)
We did a two hour cooking course here for 1200 baht. Whilst it was a lovely venue, all the messy jobs (cleaning crabs, mincing meat, etc.) had been done for us, so it wasnt as hands-on as I hoped. We made roasted duck curry, stuffed deep-fried crab and a seafood salad. The food was lovely, but be warned; we met people on the train back to BKK who said they had done a day-long cooking course in Chiang Mai for only 800 baht. Perhaps a good idea is to go to the SITCA restaurant for lunch, and do a cooking course somewhere else.
A place as touristy as Samui naturally had other cuisines on offer. Despite my insistence at the beginning of the trip that I would eat Thai and nothing but Thai, I broke down on Samui and tried some Italian restaurants on the island.
Vecchia Roma (or was that Vecchia Napoli?), Chaweng Beach
Run by Italians. We made the mistake of getting pizzas here, but should have known better; if an Italian restaurant has a pizza oven, it will be right out front where everyone can see it. This one didnt. However, the salad was great, and the home made bread and chilli dipping sauce were lovely. I suspect their pasta dishes are good. Just dont eat the pizza. You really need a pizza oven to get a good crust.
(That said, we ate at Via Vai, which had an oven, but the pizzas were still disappointing)
Olivios, Chaweng Beach
This was my romantic night out with my boyfriend. Probably my best meal on the island, too. This is an upmarket Italian place directly overlooking the sea. We had linguine with fresh seafood and a risotto with mushrooms and duck. Both were lovely. Despite the upmarket restaurant, the bill was terribly low.