Hi Linda,

I would go with Kata Beach . Even though Nai Harn is lovely, I would find it a bit isolated. If you want to escape the crowds, stay at Kata Noi, the beach wont be anywhere near as crowded there, even in high season.
Having said that, if you want a really quiet location with just a few restaraunts and bars, then Nai Harn may suit. I just prefer the balance at Kata, plenty of choice without the craziness of Patong and the beach is truely stunning.
As for Railay, you will be hard pressed doing it in a day, even by speed boat, it would be a 3 hour trip each way after a 45 min road transfer. Far too taxing for one day.
Consider flying Air Asia to Phuket,staying a few days and then transferring by road to Ao Nang/ Railay and flying out of Krabi, if you really want to see RAilay.
I will be there in a few weeks so I will let you know whether Railay is all it is hyped up to be. I have been getting mixed reports about all the constuction going on there but we decided to check it out for ourselves. We are also going to Phi Phi for 6 days as we have done Phuket to death over the past 10 years and want to try something same-same but different.

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