I have been researching very hard but can't find the answer. I googled Nathan Street and found that there are a lot of double decker buses that looked like public transportation. We're an older couple, so instead of walking it, I thought about just taking one of these buses to go up and down the street. Does anyone know what number to take, if any, and how much it will cost? Also, do we put the money in the machine when we board the buses? I think bus 21 or 21A does part of Nathan but when I looked at the citybus website, it says the fare is $37.00HKD. I believe this is only if you go to airport, because it can't be that expensive to ride just 2 - 3 KM on Nathan. Please advise.