If you do go through Helsinki and enjoy nature I would suggest a trip Nuuksio national park. Can be done with public transport (google for more info). If this is close to mid summer you can do it even if you are taking the late HKG departure since the evenings don't get dark at all. Of course many other things to do in Helsinki but Nuuksio is something very unique so close to a center of a fairly large city.
One other note that when booking connection with Finnair from HKG to TPE choose CX. Finnair website especially tryis to push CI options and it sometimes takes a while to get CX to pop up as an option.