for diving when in Phuket go to Thailand Divers (sunrise) on the main one way sysytem acrss the road from Jung Ceylon shoppoing centre - the guys are great and know the best sites - before the others arrive why not do a day trip or even a couple of days in river kwai country where you can see the obvious - but also the tiger temple - waterfalls - train market - elephant treking - biking - quads etc etc - and or chaing mai / rai great places to visit

jax is right though I think you need to cut down the travelling in the second two weeks maybe stay in phuket longer and do phi phi by boat maybe combine that with the diving