Thailand's very good high-way system encourages traveling by road, and there are air-conditioned or open-air buses reaching all part of the kingdom from Bangkok.

In Bangkok are three major bus stations:

The Northern bus station, or Mo Chit, serves Northern and Northeastern Thailand; it also has a number of buses to Trat and Pattaya on the Eastern seaboard. It is a HUGE bus station, with hundreds of counters and stands where buses leave from and arrive at all hours of the day and night. Unfortunately, it is not within walking distance of Mo Chit metro station. There is an air-conditioned waiting hall with restaurants, convenience stores, bookshop, left luggage service, shops. It is an overwhelming, confusing place to enter, with thousands of people, signs mostly in Thai: you might have the impulse to run away. However, there is an information counter where you can ask the counter number to your destination, and as there are very few foreigners in the crowd, cashiers easily spot them from a distance and show them which counter to go to. 

The Eastern or Ekamai bus station serves destinations on the Eastern seaboard, including Trat (for Ko Chang), Ban Phe (for Ko Samet), Rayong and Pattaya. Buses to Aranya Prathet, however, leave from the Northern bus station! This is a small, run-down bus station with only a handful of counters and stands. There are food shops and a 7-eleven, but no air-conditioned waiting area. The biggest advantage of this place is that it is right next to Ekamai skytrain station, almost in the centre of Bangkok. Lots of tourists heading for the beaches in the east actually do make the effort to come here.