I loved the Full Moon Party, but once was enough. Afterwards I'd prefer to stay somewhere a bit more quiet and Haad Rin Beaches are all about parties. So if you don't mind staying up late, constant noise, music, drinking etc - you're going to love this area!
To be honest, any spot in Ko Phangan you choose, is probably going to be great for snorkeling. The only difference is an access - if it's hard to get there it's not going to be crowded.
One of the most breathtaking views in Vietnam. I took the train from Danang to Hue (3hours) and even though it was rainy and foggy, it still looked beautiful. The hills of green meeting blueish waters breaking on red rocks and yellow sand beaches... Don't miss it!