I agree with Chiang Mai — book a hill tribe tour. It’s amazing.
I also recommend the Full Moon Party (or half moon, or black moon — best is Full Moon though) on Koh Pha Ngan, a Thai Island right above Koh Samui (30 min ferry ride north). Every backpacker you meet will be headed there for the a few days before and after the Full Moon. It’s a can’t miss spectacle.