Khao San Road is a short lively street in central Bangkok. This street is full with pubs, bars, restaurants, street stalls, and so on. It is also near the historical sites of Bangkok. So, many tourists choose to stay there. But the downside of this road is it does not connect with any express train such as BTS Skytrain and MRT Subway.
Siam Square is a meeting place for teenager. It is a place where you will see groups of high school students, university students and also office workers hanging around. This area connects with BTS Skytrain and also luxury shopping malls such as Siam Paragon, Siam Center, Siam Discovery. Besides, from Siam Square you can walk to MBK Center.
If you stay at Khao San Road which does not connect with BTS Skytrain and want to visit Siam Square or the shopping malls around, here are the options.
The distance between Khao San Road and MBK Center is about 5-6 kilometers. It takes around half an hour or more depending on the traffic situation and the taxi fare should cost around 60 baht or more.
You can take bus no. 15 and 79 in front of The Government Lottery Office, Ratchadamnoen Road. It will take you to Siam Center which is opposite to Siam Square. Actually this is the cheapest option (less than 10 baht), but I do not recommend this option if you are not familiar with Bangkok’s area and could not speak Thai. But you can try!
Hope the information helps