I'm in HK currently.
The airport MTR express will take under 30 minutes each way and is pretty frequently serviced. You're taken to central station which is very short walk to the Island Line metro, from there you're just 3 stops away from Causeway bay (plenty to do there). Alternatively, instead of getting the Island line metro you could hop on a star ferry (worth doing in itself ) to TST.
In short, if you choose to go to HK you'll have plenty of options in a 9 hour window.
Edit: Bring an umbrella if you decide to go to HK, the heavens have opened.