If you've never been to Hong Kong before, do pick up a tourist map at the airport -- they're available for free (published by the Hong Kong Tourist Association) at various kiosks in different sections of the Arrival Area. The places that tparker mentioned are all indicated on that map.
Also, to expound a little further on tparker's great answer -- the Peak Tram is well worth the visit if you've got great weather. If it's rainy, the view from the Peak becomes really dismal (you can hardly see anything!), and that kinda defeats the purpose of going there in the first place.
The ferry ride is enjoyable if you've never done it before, and it will give you an opportunity to take photos of the Hong Kong skyline. I personally like it best in the early evenings. It's cooler and the cityscape becomes real pretty when all the lights come on.
If you intend to do a little shopping in Tsim Sha Tsui, do keep in mind that the entire area there is a tourist trap. Be prepared to haggle like crazy before you buy anything (consider it part of the fun). I remember once stepping into a store to look for an electric adapter for my laptop. The initial price they gave me was HK$75. The adapter wasn't exactly what I wanted, so I decided to leave without buying, and by the time I was walking out the door, the salesman was offering the adapter to me for HK$20!
Do keep in mind that the airport is large and that boarding gates usually close 40 minutes before your flight, so I'd leave ample time to get back to the airport, check in, go through immigration, and board your plane. If you happen to get back with enough time to spare, you can always browse the many duty free shops to kill a little more time.
Hope you enjoy your layover!