Ang Thong National Marine Park, Samui
Instead of spending your days lying on the beach in Samui (as tempting and enjoyable as that is!), book yourself on a boat trip to visit the Ang Thong National Marine Park. Not only is whiling away your day on a boat a lot of fun, the scenery is absolutely stunning!
One of the beaches you stop at is Koh Mae and clambering your way up the viewpoint is a must - I promise the view you get is worth the pain on your ascent! But perhaps take some trainers with you; flip flops were not adequate footwear!