With the Great Palace, Wat Phra Keo and Wat Pho to name only the 3 most famous monuments, Bangkok is an essential stop for those who want to discover the Thai culture and understand the place that the Buddhist religion occupies in the Life of the Thais. Lovely parks, day or night markets, klongs, Chao Praya river, vendor stalls that reduce the sidewalk to a narrow alley, people who smile at you, temples, tuk-tuks, nightlife that shows the best and sometimes the worst of Thailand, massage ... You are in another dimension. Sometimes exhausting, Bangkok, if you let it time, will seduce you.
The best moment to go to Bangkok is in November, December, January and February. The least favorable months, due to the monsoon, are from June to September until mid-October. September is the month when it rains the most, there can be some flash floods in some parts of the city.
You can bargain prices for a lot of things in Bangkok especially for tuk-tuk or motorcycle taxis. For taxis, prefer those with a taximeter and insist that they use it. This is the best way to pay the fair price.
Be careful, some tuk-tuks drivers around Wat Pho or Grand Palais may tell you that the temples are closed and propose to take you elsewhere. In the vast majority of cases, nothing is closed, they just want to drive you to stores that reward them with gasoline vouchers for each customer they bring. Sometimes you'll even end in jewelry store where you will be scammed.
To get to or from Suvarnabhumi Airport, the best option is to take the Airport Rail Link from or to Phaya Thai (served by the BTS) or take a taxi. To get to Don Muang Airport, where most of the low-cost airlines depart and arrive, there are express buses departing regularly from the airport to BTS Mo Chit, Bangkok Northern Bus Terminal or Victory Monument. Don Muang is also accessible by train but trains in Thailand are regularly late ! A free shuttle bus service runs between the two Bangkok airports (from 5:00 to 22:00, it takes around 1h30 , more info HERE)
All information about Transportation in Bangkok are HERE.