I just have one more tip for you:
When travelling from the airport to the island by the Airport Express train, be sure to buy your ticket at the customer service center in front of the station and request for SAME DAY RETURN ticket. You can also choose that option by using the ticket selling machines.
SAME DAY RETURN ticket works on the day specificed, and charge HKD 100.00/ USD 12.85 for the journey from the airport to the island, and your return journey from the island to the airport is free. Remember to keep the SAME TICKET, in order to travel your return journey for free. There is no gate at the Airport Station, and you just board the train after you bought your ticket. When you arrive at your destination: Hong Kong Station (The station on the island), you just need to insert the ticket into the gate when leaving. Insert the SAME ticket again into the gates when departing from the Hong Kong Station (The station on the island) for the Airport Station.
This method will save you USD 12.85, because if you buy a single journey ticket, you need to pay AGAIN to board the train back to the airport from the island. So, the total cost will be USD 25.70 for the round trip. With Same Day Return ticket, your total cost is only USD 12.85!
For more info, you can visit the airport express's fare chart website:
http://www.mtr.com.hk/eng/train/ae_ticket.htm