A few pointers in detail for your reference -

The currency exchange booth at HKG is just outside the arrival hall, after exiting immigration control and custom - where the MTR Octopus Card and/or AE Passes can be purchased. The conversion rates aren't great (about 5% premium/reduction) compared to licensed exchanges easily found in the city's tourist area like TST & Central, or walk into any of the HSBC or Standard Charter Bank (Passport/ID's required nowadays plus a transaction fee, as I recall) - but the airport location is open around the clock, 24/7 whereas the ones in town are NOT but they offer better, official rates - especially for major ones like RMB, USD and Euro's.

For a short 24 hours or less stay, you can exchange $100 USD per person which should yield about $775 HKD at most places, but maybe $725 HKD at the airport (I just don't recall exactly nor can I find my last receipt - FT's/locals, please help with your input on this) - that amount will be enough to buy the AE Card or Octopus Card ($400 with $350 available for use and a $50 refundable deposit) - it will cover regular MTR rides/day pass, express bus, ferry, island tram and Peak Tram, plus you can use it to debit buying incidents at numerous 7-11 all over town for things like English newspaper, soft drinks & bottled water, gum, pocket-sized paper tissues (tip/hint: it is very handy for lots of things) and freshly-baked goods, tea & coffee sold inside MTR concourses for folks on the run.

For dining at upscale restaurants, you can always charge it to your CC or use cash (HKD only) once you get into "town" (unless it's a Sunday as banks are closed & you will have to find currency exchange that's open) and exchange more local currency. Taxi drivers only take local bills & coins as only a handful of them are equipped to do CC transactions.

Another option and the net effective exchange rate is just as good as using the exchange is to find the next bank ATM - we've used both HSBC and Citi(bank) ATM's and just withdraw $$$ in local currency using 4 digit PINS, no Passport needed - fast & easy (did this once inside the Kowloon Tong MTR/Rail Stations' concourse) - just a small foreign transaciton fee for the conversion & non-bank ATM usage charge (which I'm cool with, as you are saving time & not standing in line.) That's been our strategy for visiting HKG - get USD $100 exchanged into HKD in the airport for a running start, and withdraw anothe $100 or $200 for next day or two as needed for daily use, and charge other items on CC as needed (usually we call our bank/CC to alert them about our travel plans to avoid problems with fraud prevention.)

At the end of the trip, I just ziplock bag the unused bills & coins until the next time (and airlines like CX give out envelopes in-flight, encouraging pax to donate/contribute their leftover loose changes to worthwhile charities instead of being dumped & taken home for the cookie jar)