All suggestions are good; here is a little more info ...
The Octopus card is your friend. Generally no change is given when boarding busses so the ability to use the card is a godsend. Note that you pay a deposit with your card and you can get a refund of this (and unused credit) when you have finished (less a small fee if you have card for less than 1 (or maybe 3) months).
Day return on Airport Express is HKD100; don't need octopus card here really; just buy at airport.
The main bus station in Central is at Exchange Square. It's like a huge road level 'garage' underneath a building with a dozen or more separate island stops. Buses queue up and come in and out of here all the time during the day. Identify your bus stop using the #number or ask at the customer service booth.
Bus #15C from Central station to Peak tram lower station. Peak Tram now costs HKD22 (~AUD4) oneway (IIRC no Octopus). Check out the views north of the peak Galleria out across Central, Admiralty and the mainland.
Take bus #15 from Galleria basement back to Central; ensure you get top deck front row seats to maximise experience as you travel down off the peak. (Unless you get vertigo, in which case what are you doing at the peak anyway?) Wait for the next bus if you need to so to get to the front of the queue.
Catch Bus #6, #6X, #6A or #260 express from Central to Stanley Market. The 260 is the quickest but costs the most.
Stanley market is a bit touristy these days but ok.
When you have finished with Stanley you might like to visit Aberdeen and its floating restaurants, take #73 from Stanley. (#973 will get you there as well but somewhat more for the fare) To skip Aberdeen, #260 from Stanley will take you straight back to Central.
Bus #70 from Aberdeen to Central is one of the better ones of the many options to get back to Exchange Square.
Just for fun, walk from Central across Connaught Road using the overpasses to Des Voeux Road and take a Tram to the Western Markets for a look around there (Just about any tram will get you there, including "Western Markets".) It cost HKD2 per ride, payable on exit. (use Octopus) Take a tram back (any east bound tram will do).
Walk from Central to Star Ferry as mentioned and head to Kowloon.
More info on Buses (including times / fares):
Citybus - Bus Route Search.
Peak Tram:
The Peak Hong Kong
I'm there in two weeks taking family for Whirlwind Visit including Disney.