A whole month- I wish I could do that! Are you purely heading to the west? I can recommend Kanchanaburi. We only went on a day trip but a lot of people stay for a few days. Stunning scenery and of course you can visit the Bridge over the River Kwai, the POW camps and the cemetaries. You can also go sailing down the river on rafts which is very popular.
To help those who may have recommendations- what sort of holiday are you after- adventure, site seeing, relaxation or a bit of everything? And are you a budget traveller or looking for a bit of luxury?
Have a look at the sticky on recommended websites at the top of the page- there are some very informative ones for Thailand.

Thansk Fiona. We're after a bit of everything and would prefer to stay in nice hotels , my days of backpacking are well and truly over!

As to the type of holiday, well a bit of everything really. Plenty of nice beaches to relax on, but as we are going for so long, it would be a shame not to do anything else.

As for areas, we don't really have a limit, I mentioned the west as that is one area we have been and definitly intend to go back, but getting anywhere from bangkok is pretty easy.

Have you considered flying north to Chiang Mai before hitting the west coast? Easily done with a month you lucky thing. Anyway, what about Phuket? From there you can visit Phi Phi and Khao Lak.

Great time of year for Phuket.

As you have a month I would recommend doing some of the North of Thailand as well...beautiful countryside. I particularly like Mae Hong Sorn and also the Golden Triangle area.

Thanks, considering Vietnam for a few days and maybe also Chang Mai?