It's the first time I've traveled to another country. I only visited Bangkok but I really appreciated the culture and people here, somehow strange but somehow familiar also. The only thing made me uncomfortable was the traffic jam, and my traffic-sick disease haha.
This is much like a business trip than a vacation, because we have to look for suppliers and products for our upcoming clothing store. So most of the time we wandered around the markets and malls inside Bangkok, transporting using mainly Metro railway and taxi. We were not familiar with MRT because we didn't have it in Vietnam at that time so we preferred taxi. However, that was not a great idea because you might get stuck in the traffic jam for hours on the street, then the suffer limit was reached and there was nothing better than getting out of the cab and walking.
There is no significant architectural construction in Bangkok for visiting beside Watpho, the Thai royal palace and the signature temple/pagoda architecture. Please don't forget to check the calendar for Thailand public holiday because there are some disadvantages on such days, like alcohol is prohibited, Watpho or temples and pagodas don't allow foreigners or non-religions, etc. And don't forget to visit the animals in Safari World Bangkok.
The main thing you should try in this city is street food. Keep that in mind. And pomegranate juice. I promise you will not regret.