You can take the MTR transit bus from most larger hotels (and just go to the nearby larger hotel from your smaller hotel if needed) to the MTR downtown (Central) or Kowloon. It's free. You can then checkin at the city checkin and go about your business until it's time to go to the airport. Charging works as above, by the time you have checked in you have an airport express ticket also. You can then go out and use the MTR to get about the city until you return and catch the airport express.
It truly is a civilised way to travel. For City to Airport travel with bags, HK has it sorted.