I'm not a diver so I can't help you on this point
I was in Nai Harn last year : really nice beach, appreciated by Thai people, some friendly restaurants, quiet, not far away from Chalong, Phromtep Cape, Rawai... but I think you should rent a car (500 - 600 bath/day) or a motorbike to visit the island.
This is the place where I was last year and I negociated it for 30.000 bath for 23 days (house with 2 bedrooms)
http://www.houseinphuket.com/houses/STR/p0006/03/
Laguna is in the North of the Island. No nightlife. I don't like too much but many luxury hotels there.
Easy to take a plane to Chiang Mai (I think the cheaper will be airasia.com) and you will have a stop in BKK if you come from Phuket. You can also take a night train from BKK to Chiang Mai, cheap but a little boring as you see nothing. The guy in the train will make your bed at 8:00 PM, then you can only sleep.
Hua Hin is the place where the Royal family has holidays, I have a friend who has lived there a few months : he told me it was quiet but a good place to relax.
If you have never been in LOS, don't forget to visit BKK where you will find Royal palace, temples .... but it is noisy and for me 3 days are enough