Although justly famous for its Full Moon Parties, Koh Phangan has a lot more to offer visitors than all night dancing. There are few day trip options from Koh Samui, apart from sailing cruises, so at least an over night stay is recommended.
Koh Phangan is at least ten years behind Samui in terms of development, which lends the island a lot of charm, and some would say it's more picturesque than Samui itself. The main tourist beach Had Rin is busiest on Full Moon nights, but is well worth a visit the rest of the month too for its chaotic jumble of bars, clubs, restaurants and handicraft shops. The locals - expats and Thai's alike - are a friendly bunch and help make the atmosphere a lot more laid back than frenetic Chaweng. Their attitude is that every night is a party night on Had Rin, which pretty much sums it up.
The main selling point for the rest of the island is pristine undeveloped beaches, with only a handful of bungalows to disturb the peace. The most upmarket of these is Tong Naipan, thanks to the presence of the island's most luxurious resort, Panviman. At the other end of the scale, Had Salad and Had Kuat both have very cheap and basic bungalow resorts and attract many long stay backpackers. Had Tien is very shallow and pebbly, but is home to the Sanctuary Spa and Wellness Centre which offers a range of fasting, yoga and treatment packages and has an excellent vegetarian restaurant