Although we had hoped to stay on Koh Pha Ngan the whole time, I'm glad that we stayed on Koh Samui Island.
We stayed on Chaweng beach, which is supposedly the nicest beach on the island. You could keep walking along the beach for a loooong time, passing by beautiful little hotels, restaurants and bars.
If ever you're in the area, definitely check out i) The Library - it's an expensive place, but happy hour has two-for-one drinks and it has a great vibe and beautiful landscaping; ii) Ark Bar, which is a chill bar on the beach and turns into quite the party pretty much every night - a good place to meet people; and iii) the massage huts at the right end of the beach (when you're looking out at the water).
I would also recommend eating at the little Thai cuisine places along the first street that runs parallel to the water, and try to find the night market - it's on a side street, but has amazingly cheap and delicious eats!
I would def suggest doing a couple of nights on Koh Samui, it's a super relaxing and beautiful place. If you book early enough (which we didn't, unfortunately) you can find some pretty reasonable huts on the beach or a cheap hostel off the beach.
Or splurge at a higher-end place.
Either way, there's plenty of corn on a stick to go around ;)