Done this a few times. I take the Airport Express to the Hong Kong Station where you can check in for next flight, and check luggage if needed/desired. Walk over to the mid-level escalator(s), Head up to Elgin street, find a nice restaurant for lunch. Afterwards walk down a bit. Hit the Peak Cafe for some people watching.
Depending on how long my layover is, I'll eventually head back to the airport to take a shower at a lounge and settle down with some champagne to wait for my flight.