Hi Emma, Ask 10 people and get at least 10 different opinions!
Your planned time of year is good- not the main monsoon season, so just hot and humid!
I spent a lot of time in SE Asia a few years back. Unfortunately I can't remember the names of alot of the "special" things, places, cafes I came across, so my advice is kind of generic. Yes you will be rushing, but you'll get to see old buddhist temples and ruins, the splendours of Angkor, Vietnam war relics, French colonial-Asian style, rice paddies, mountains and jungle, and some of the best food in the world. In my humble opinion, the best bits of the places you are aiming for are:
Bangkok- the Grand Temple / Emerald Buddha; Chatuchak market; street food. And yes, you are right, Thailand is a great place to start as so many backpackers head here first. I remember having a great lunch at a random street food market for workers, somewhere around the Siam Square shopping centre, it cost like 20B, but of course very spicy!
Ayuthaya or Sukhothai (never made it to the second)- but have great ruins: However in my opinion I'd not do either as you are going to see all the Angkor temples... Tony's place in Ayutthaya was a nice place to stay.
Skip Chiang Mai and go on to Pai or even Mae Hong Son. Pai (Golden Hut oin the river was my favourite place to stay) is a lovely little village by the river, it's 'backpacker touristy', but a nice place to relax, learn to cook (Pai cookery school at the garden of the Glass house), can do day walks from here, hire mopeds etc. Or if you go on to MHS there's a great guy who ran a company (from the steps of the post office in 2007!) and you go with them well off the beaten track for some seriously good jungle trekking with a night in a local house. Be prepared to go waist-chest deep in the river several times on the second day, and have your guide get you to roll your trouser legs up every so often to do a leech check with his machete at stand by! Not an elephant ride or bamboo raft in sight like all the treks from Chiang Mai. It's 8 hours by winding road on local transport, but Thai used to have flights out to the airstrip there.
Vietnam: I loved it here, true there's a bit of hustle and hassle, but it's just the way it is. And you really do just have to step out into the road to cross it, walk slowly and steadily across and the drivers will avoid you. It's well worth going to as a contrast to the rest of south East Asia- more Oriental. Also with the French influence you can get some interesting 'fusion sandwiches' involving French bread and, well, sometimes it's better not to ask... The local food markets were some of the best I remember seeing in SE Asia.
I loved Hanoi (stay in the Old / French Quarter) with it's lake, old chinese college (can't remember the name), Bia Hoi bars (illegal homebrew pavement bars, great way to meet locals!), street stalls selling noodles but you'll feel like you're back in nursery school with the tiny plastic chairs... I felt that despite it being touristy, a few tens of dollars (30-ish US in 2006) for a few days in Halong bay on a junk / Cat Ba island was worth it. Waking up on the deck (drag your mattress from the cabin to the top deck) with the limestone islands in silhouette is an awesome experience. This makes up for the touristy caves you'll get taken to, and the surly staff you might have onboard!We also had a good group so that helped.
I really liked Sapa too, I climbed the mountain which was fun, but generally it was a nice place to wander round, rice paddy fields ... and the Cat Cat (?) hotel/guesthouse had great views from its terrace high up. It might also be an ideal place for you to relax as by now you will have been on the road for a few weeks, in lieu of a beach place.
Hue is good for a day: the Imperial Citadel and emperors tombs, we went out to the DMZ but it was a long day and I didn't think it worth it as much of what you "see" are places that used to be there, so how much you get out of it depends on your guide. Ours wasn't very good. If neither appeal, you could skip this.
Hoi An is a must- beautiful town (UNESCO WHS), and you can get your very own tailormade clothes here.
I too found Saigon to be my least favourite place, for the reasons outlined above. However if you're here for a day the Independence Palace and the War Museum (formerly known as the American War Crimes museum!) are worth seeing. there was a large market for cheap clothes should you be sick of the ones you are wearing by now. The Cu Chi tunnels are a fair trek out for what is a display of some man traps, guns and a crawl through the tunnel.. but it was of some interest.
Kitsch Dalat in the highlands was a fun detour (including the huge concrete chicken given as a present to one girl by a besotted local!), if only to escape the summertime monsoon heat and humidity.
Skip the south east coast, especially as you are not interested in beaches. There are better elsewhere in the region.
I wholeheartedly agree that the food is wonderful- and it genuinely did vary from place to place (at least there was a distinct local speciaility in each region). If you were short of time, I'd recommend the northern stretch down to Hoi An. As well as the train, the buses weren't too bad, considerably cheaper than by rail back then and they used to operate a coupon-ticket system, similar to a bus pass, for the route you wanted to do.
Cambodia- well you have mentioned most of the places I went to! I didn't get any further away from these as much of the jungle areas are off limits in monsoon season.
Phnom Penh is a complete contrast to the north. We felt a little guilty going to S21 prison and the "Killing Fields", and walking through all the galleries, cells etc... a bit voyeuristic. But every local you spoke to wanted to know you'd been. It's a tragic, but important part of their history.
When you get to Siem Reap there are various options for how to 'do' the temples. It all depends on whether you want a guide, or have a good book and like to explore alone... a girl I met on the way from Vietname and I hired a couple of bikes for three days (that was the length of one of the entry options) and did our own thing. this was great as we were able to spend as much time as we wanted at places, living out our Indian Jones/Lara Croft fantasies, revisiting when the rains had gone, or stopping in the shade to watch monkeys play... It also means if you hit a place as the tour buses arrived, you can always 'sit it out' with a coconut water drink at a little tea stall and wait for them to go. The children at their mother's tea stands will look after your bikes for the price of a drink when you return or a post card. From memory, what the LP guidebook said about times of day for tour groups at the larger temples was fairly accurate.
There might be some inspiration on the webpage I did for that trip. South East Asia would be from June-October 2006:
http://blogs.bootsnall.com/Nicks-world-tour/
All in all, you'll have a great time. You mention that you're new to the travelling thing, well you've picked a good place to head to first. There will be lots of others around, whether you see this as a good or bad thing is entirely down to your perspective, but one great thing is that there will always be someone who has been to where you are headed next... especially at the guest houses in Bangkok. Their knowledge beats any guidebook or travel agent. Most of the things you want to do are possible independently, apart from trekking guides, the trip out to Halong Bay etc. Bus and train tickets are sometimes more easily obtained through the place you stay or a travel agent if the station is a ride out of town, the commission they charge is often worth it. Remember as well Vietnam visas need doing in advance, as you're away for a short trip this may be worth doing from home as it used to take a while to get one issued en-route. Flying a short sector may be an option as well. Reliable airlines in the region are Thai, Bangkok Air, Vietnam Airlines. Air Asia (budget based in Kuala Lumpur) were just getting a grip on the northern part of SE Asia a few years back so should offer a fairly dense route network now.
Report as inappropriate