I would recomment Khao Lak for a quiet few days. It is about 100km north of Phuket and has some lovely beaches with a small township. It had a lot of damage with the Tsmanui but has not recovered well. We stayed at the Le Meridien which is 10 KM north of Khao Lak but there are plenty of well priced places closer. In regards to Koh Lanta you definetly would be better off flying into krabi. If you want more remote fly into Krabi and head to the islands in the Trang area or fly into Koh Samui and head to Thong Nai beach in the north east region. No shops to speak of just little resorts along a wonderful stretch of beach. We stayed at The Santhiya which has a land and boat transfer from Samui airport 45 minutes. It is a luxury resort but well priced.
Here is a link to some of my khao lak, Samui and Koh Phangan photos for you to have a look at.
Picasa Web Albums - Marie - Thailand 2006