Where to go?

Hi,

My girlfriend and I are flying into Bangkok in mid December, then going to visit River Kwai/Kanchanaburi, followed by Chang Mai.

We would like to finish by visiting a couple of the thai islands, haven%26#39;t decided which yet.

We have about 10 days spare in between, some of these we would like to go to Cambodia (Siem Riep and Phnom Penh) but not sure where else to go.

I have looked at Laos and Vietnam but not convinced about either. Does anyone have any advice on where we could go.

Open to suggestions, but don%26#39;t really want to be sitting around too much.

Any help or must sees would be most appreciated

thanks