There are plenty of things to do in Koh Samui, it just depends what you want to do during your short vacation. You can party at night and just lay down on the beach during the day. Unfortunately, Koh Samui has now become a bit commercialised but still a good place to go.
There are plenty of accommodation in the island, just depends on your budget. Chaweng Beach is where the main action is. I personally stay at Coconut Grove Hotel just behind the Pizza Hut in Chaweng. It cost 800 Baht for an double bed air conditioned room and the hotel goes right onto the beach.
A good way to see the island is by scooter, it will cost you around 180 to 200 Baht for one day. Its pretty fun!! If you are a party animal, time your visit when the Full Moon Party is on at Koh Phangan. This is around 25 minutes by speedboat from Koh Samui. Basically, its a big party on the beach when there is a full moon. I have been a few times and it always been pretty good.
The only thing with Koh Samui is that Bangkok Airways owns the airport, therefore, they have a monopoly on the flights and its always expensive. The alternative is fly to Bangkok then catch a night train to Surat Thani which will take overnight from there its anothe 3 hours ferry ride to Koh Samui. The train ride is pretty fun you will meet a lot of people.
Hope this helps.