Let's say you land somewhere at 0630 and your roster shows about 28 hours rest before your next sign-on at about 1100.
You may do whatever you like in that intervening period - as long as it is legal and unlikely to bring the company into disrepute. Go bungy jumping, go to the library, go get some cookies from a cafe, hire a car (or an aircraft) and go for a big day out somewhere, or drink as much as you like as long as you meet the requiremements of the HKG CAD AND the requirements of whichever country you are in.
Those 28 hours are yours - there is no need whatsoever to be on call or to answer any message lights in your room. With respect to the AFTLs, you should get some rest at some stage so that you are fit for your next duty, but how you do that is up to you.
If, and only if, the company rep meets you on the aircraft on arrival and says: "Sorry Ol' Chap, we need you to PX (in two hours) to the next door country and then operate XXX service back to HKG tomorrow" then you are fat out of luck as long as what he is proposing is legal in all respects - esp. with the amount of rest being at least as long as preceding duty period.
Also, should you be "disturbed" in your hotel room while you are having rest they should give you another 10 hours before your duty. Any less than that and you are doing them a favour.
Other scenarious also considered....