For cheaper hand luggage storage, make your way to Tung Chung (bus #1) and put it in the storage lockers at the lowest level of the CItigate shopping centre where the bus stops. You will need an octopus (buy at the MTR adjacent - also needed for transport anyway). You can then catch the MTR into Kowloon or Central (much cheaper than the airport express direct from the airport). You have plenty of time to visit HK.