Outside - nothing spectacular but inside of the temple is really nice. The walls are colorful and Buddha statue is golden. It's totally different from other temples I've seen.
Wat Phra Kaew is in the area of the Palace, you can visit all at once. Very pretty building and the surrounding gardens make the place chilled and natural.
One of the most popular places to visit in Ko Samui. All the tours stop there, there are always many many people. It's quite pretty, of course it's a must see of the island. The statue is interesting. Go early to avoid masses...
The red hall in this temple is outstanding, like nothing I've seen in Ko Samui. Very nice place, there are many possibilities of taking great pictures.