Thailand, I have not been to Koh Samui, but I would recommend Koh Phi Phi or if your into diving Koh Tao.
also Chang Mia, there is mountain trekking there, one day, two day, and three days as far as I know, but probably more if your interested. the night market is also worth a looksee as there is a lot going on over a large part of the town, but gets a bit repetitive with the goods that everyone is selling.
Cambodia, Angkor Wat, and there is a park in south eastern Cambodia near the border to Vietnam, the town is called Kampot, you can take day trips up into the park and see some very old ruins from when the French were in the area, breath taking views, was an amazing day trip!
Vietnam, Dalat was amazing as well, there are some older guys doing motorcycle trips in the town, they have one day or more depending on what you want, are very informative and take you all around their city, into peoples homes for lunch and shots of home made whiskey, and some beautiful waterfalls, among other place. it cost about $20us and was worth much more then that!
Laos, I was there for only 14 days which has been such a disappointment for me that I'm going back this year for a month, the only town I really stopped in for long enough to take in some sights and smells was Vang Vieng, we did the river tubing, which is a booze filled ride down the river in a tube, and a two day trek with some locals, stayed over night in the bush, ate a typical Laos dinner, and went on another tube ride, this time in some underwater caves, which was quite exciting, and an amazing but strenuous hike for probably ten hours in total (some on the way to camp, set up camp, hike up to some falls and back down for dinner, sleep, hike the next day around a village to get to the underwater tunnels.
I agree with taking flights, it will save you HUGE time and hassle, as most everyone goes by the slower time, I met a guy travelling with about the same time line as you and seeing the same places, he was able to see the major sites but did feel a bit rushed.
one suggestion i have is get a map of the four countries and pick out a site that is MOST important for you to see per country, and then make a sort of route based on that. we found that very helpful in planning our upcoming trip!