1. March is a good time to visit the north apart from the potential for smoke pollution due to the "slash and burning" of the fields and forests which can be pretty bad at that time of year. Doesn't happen every year though so it is just pot luck really. NB. We travelled from CM to Luang Prabang in 2007 at that time of year and the smoke was really bad. LP was almost as bad as CM and in the end we decided to cut our losses and fly back to BKK ( imagine that, going to Bangkok for clean air!!!)

2. BKK to SR by train and bus is easy enough but the border crossing can be a PITA. The drive from Poipet to SR is fairly uninteresting though. You will save money this way but it does take time and wold eat into your holiday time. From memory the train times require an overnight stay in Aranyabprathet, not the best place in Thailand.

3. No idea!

Thank you. Love how fast this forum is. Maybe trains aren't a good idea, since I will end up paying for an overnight stay. Btw, do you think my itinerary is reasonable?

Check the Bangkok AIr website for info and pricing on the Discovery Pass. Going overland means you spend a lot of your sightseeing time on the road, basically a full day each way for each destination.

Your itinerary is reasonable as long as you fly. I would suggest going from Siem Reap to Phuket (the plane stops in Bangkok, but one flight a day from SIem Reap to Phuket is considered a single "leg" so this will save you money. Then spend your last two days in Bangkok before flying home.

with limited time, which is what you have, the only reasonable way to travel is to fly, imo..

plan looks great to me, but i would bag ayuthaya, as you are going to angkor which is far better.

pick your bkk hotel carefully..

Thank you! Do you think it's too late to plan the trip? I'm going in March.
I already have an idea about what to see in each destination, but I haven't booked any accommodation yet.

Would you recommend Halong Bay instead of Luang Prabang, due to the field burning issue?

no its not too late, but today would be a good day to do it..

If you are set on particular hotels the book ASAP but you will always find somewhere to stay. We often travel in SEA without anywhere booked but can always find somewhere to stay. Places like LP will be busy at that time of the year so always best to book in advance. As for your itinerary, it look fine to me. Like bob, I would suggest forgetting about ayuthaya. There is more than enough in Bangkok to keep you occupied. Indeed you could double the amount of time there and still not fit in the good bits!

The smoke issue in the north is a hard one. LP is a special place, although a lot lot busier than it used to be years ago, it has lost a little of its charm for me. It is however, far preferable to Halong Bay. The smoke was very bad in CM when we visited in 2007 and a little better in LP but still not great. CM was bad again in 2012 but I don't know how LP was affected. There is no real way of predicting what will happen from year to year. There used to be a good Thai website detailing the pollution stats for the region but I can't seem to find it anymore ( perhaps the tourism authorities have pulled it? If Hanuaman reads this he may be able to she'd some light..).

If you did want to include Vietnam, then Hanoi would be my first choice with a possible side trip to the beautiful Mai Chau valley, or maybe Sapa, but that is an entirely different trip. Some photos of these places on my blog
http://blog.travelpod.com/travel-blog/candcthai/3/tpod.html

Thanks a lot crellston. I am not sure I can handle 6 flights within 3 weeks, and LP looks a gamble with the smoke issue. I'm thinking of reducing my trip to 12 days.

Here's my revised itinerary, tell me what you think:

Bangkok 4 days
Siem Reap 4 days
An island (probably Ko Phi Phi) 4 days

If you were to choose one island, what would it be? What would you recommend for a first trip?

Ko PhiPhi is a fine choice.

Let me suggest you add one more day to your trip so you have an overnight in Bangkok before flying home.