It is a bit of a hectic schedule, but hey, we all have our own styles of travel. I've been making short 2-3 day weekend trips to countries bordering Thailand for years with no problems. Of course, I am a frequent and repeat visitor, so these quick trips do not shortchange me of much in terms of getting to know the area. If you are in Thailand, and have the time/desire to see more, I urge you to at least briefly explore a neighboring country.

Laos, one of my all-time favorites, is very accessible from Thailand. A round trip from Bangkok to Vientiane on TG is about US$150. A less expensive route would be to fly or even take a bus (from Chiang Mai, not Bangkok) to Udon Thani and then head up to Nong Khai (one hour north of Udon Thani), where you can cross over the Mekong River on the Friendship Bridge into Laos. Another reason that Laos should be considered is that you can get visa-on-arrival, so you do not have to waste time in BKK or back home. Of all the countries you mentioned, Laos is by far the most culturally similar to Thailand - if you can read/write/speak Thai, all it takes is a day or so and you find yourself getting by in Lao! - and far different than Cambodia and Vietnam, despite it being part of 'French Indochina'. Lodging in Vientiane runs the gamut from very cheap to a couple of 4* hotels.

A second choice, for me, would Phnom Penh. You can also get visa-on-arrival at the airport, and morning flights from BKK are easy to come by. If you could do two nights in the capital, you'd have plenty of time to see the city and countryside, including some famous and infamous landmarks. Flights on TG BKK<->PNH are surprisingly plentiful, with RTs going for about US$130.

Burma (I'm generally not politically correct, but I generally use the PC term for this country) would be my third choice, only because few people go there, and it is quite controversial, making it (for me) an interesting destination. The Burmese embassy in BKK will generally provide a visa either same day, or next day. Burma has to have the most impressive Buddhist temples (apologies to all my Thai friends) in all of SE Asia. Rangoon (more PC here) strikes me as a city frozen in time... it hasn't progressed much since the British left, and is fascinating in that regard. The only problem that most travelers to Burma come up against is the so-called 'mandatory' exchange of US$300/person for Burmese FEC (foreign exchange certificates), which cannot be changed back. There is a dark grey way around this, but I won't elaborate unless pressed

I go to Vietnam more than any of the aforementioned countries (except Laos), but would probably rank it last, only because it truly requires more than the allotted time for a decent first visit.

In regards to Thai ‘tourist hotspots’ located at least two hours or more by car outside of Bangkok (so I won’t mention Ayuthaya, Kanchanaburi, Rayong, Chon Buri, etc.), Koh Samui (koh = island, and is pronounced ‘gaw’ somewhat abruptly) has turned into a tourist/resort island in the Gulf of Thailand. It is accessible by ferry from the mainland, but I would suggest a low-cost direct Bangkok Airways flight (they own the airport, TG not allowed from Bangkok. For me, and even nicer and less hectic locale is Hua Hin, about three hours SW of BKK by car. The King has one of his palaces there, and the area has become quite popular with upscale Thais.

Good luck on your trip. If you are like most visitors, you will fall in love with this area.