Ko Samet (เกาะเสม็ด in Thai) is a rather small island where you will find some of the most beautiful beaches in Thailand. The water is also normally very clear here. Relaxation is the main activity here, so spoil yourself a little and "refill your batteries" in an tropical surrounding.
It is the perfect place for hiding away from daily life stress and work. Anyhow, you are still close to Bangkok and it's shopping, night life and attractions.