i come from hk.. i can suggest you where to go ...
if u want shopping .. i suggest you go to Causeway Bay.. if u want to see HK then go to Central.. ( u can visit both Central and Causeway Bay.. they are just few stations away by MTR underground train) or if u want to see the nasty side of Hong Kong ... go to Mong Kok lol... Mong Kok is pretty dirty n messy .. =.=
cheaper option... take shuttle bus to Tung Chung Town Centre ( take Bus S1 from the ground level.. about 10 mins bus ride) and then MTR train to Central. umm about 30-40 one way.