I've been to Southern Thailand 4 times and seriously: This is the most beautiful place I've seen of all the beautiful places in Thailand. It is a group of 9 islands in the Phan Nga Province which were made a National Park in 1982. (Similan means 9)
Because the islands are made into a National Park, and are mainly visitable during the time of November until May (because of the strong monsun) the flora and fauna in this area is preserved like next to nowhere else.
(okok, there are some places similar to this - for example the islands of San Blas/ Panama, but that's another story)
How to reach it? You can reach it via speedboat from Kao Lack (around 70mins) or from Phuket.
What to do? Enjoying the beauty of nature, snorkeling and scuba diving, taking a swin in the pristine water and relaxing at the white beaches.
My recommendation: Stay one or two nights at one of the islands to enjoy the beauty without the relatively large amount of other tourists there. A lot of tourists are just doing day trips.
This place seriously is a small paradise. I don't think there is a lot more to say...