6 hours in Hong Kong

I arrive at 16:00 andleave at 21:00. I wanted to take the airport express to Kowloon Station, walk for a while, get the ferry to HK, see the beautiful skyline and then get back to the airport. Does this make sense in 6 hours? What type of ticket/card sould I buy? Do I buy it at the airport where I have to buy local currency? Thank you in advance