We have a 12hr stay over at HKIA. best place to refresh & relax?

According to your limited time, you can take bus no.A21 at Airport (Ground Transportation Centre), get off at Cameron Road, Nathan Road. It takes around 65 mins on the way. Then walk south along Nathan Road to arrive at Tsim Sha Tsui Station. There are numerous shopping malls. You can find alomost every famous brand in the world. You can also find several nice local restaurants that provides authentic local snacks. Have fun!

For the limited time, you can take Airport Express to HK Station and walk to Central. Sighteeing, shopping and dining are all available nearby. You may have a good time and then get back to HKIA in the same way. If preferring refresh and relax, it is best to stay in the airport. Pay-in lounges there can be a nice choice.