Khao San is just a short street in central Bangkok that is full with hostels, pubs, bars, restaurants, shops, and street stalls selling foods, clothes, books, CDs & DVDs, and so on, but it is one of the popular areas to stay while tourists visit Bangkok especially backpacker.
Khao San is near the famous sites in Bangkok such as Wat Phra Kaew and The Grand Palace. The downside of this road is it does not connect with any express train that connects you to major areas in Bangkok.
So, if you decide to stay at Khao San area and your flight arrives at Suvarnabhumi Airport. Here is my recommendation to get from Bangkok’s main airport to Khao San Road.
Airport Rail Link + Bus
Use the Airport Rail Link to get you to Makkasan, and then take bus number 556 to Khao San Road. This is the cheap way to reach Khao San Road. Instead of taking a bus that you might have to wait more than 20 minutes, you can also take taxi from Makkasan.
The Airport Rail Link closed around midnight and starts again around 5.00-6.00, and the bus number 556 runs around 5.00 to 22.00. So, if you flight arrives very late at night, taxi is the only choice.
Taxi would definitely more expensive, but it is more comfortable. Anyway, you should know that Taxi fare in Bangkok is very cheap comparing to other countries in the world. I would say it should be less than 400 baht. Many taxis are waiting for you at the airport 24 hours. The only problem is some taxis might refuse to turn on the meter and offer you flat rate fare which is, of course, more expensive. What you should do is not go with this taxi, and try another one.
Another great choice is to go down to 1st floor and buy a mini van ticket to go to Khao San Road. It costs 100 baht.
Hope this information helps. Enjoy traveling Bangkok!