Travel route Thailand, Laos, Cambodia & Vietnam

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I am planning on goin traverling in october 2009 for around 3 months. I have been thailand on a holiday and would like to go back there aswell as travel Laos, Cambodia and Vietnam along with spending some time on the southern islands of thailand.

I plan to fly to Bangkok and head north to do some trekking then cross over to Laos. i dont really know what there is to see in laos but would like a general popular travel route to spend a few weeks there and see the main sights before crossing to vietnam. I want to see halong bay and then make my way south ( again i would to know the main must see sights) before crossing into cambodia. Once in cambodia again i would like to know the main sights and route (lol sorry for the lack of knowledge). From the i plan to travel back into Thailand and spend some time on the southern islands.

i was look at a time scale roughly off:

Bankok - 3 - 4 days
Northern Thailand - 10 days
Laos - 14 days
Vietnam - 21 days
Cambodia - 14 days
Southern Thailand & Islands - 27 days

I am flexible with how i can travel and willing to make some train and bus journeys but would like to fly where suitable to save time.

Any help would be mush appreciated Thanks.....

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