Take bus N11 to Hong Kong station, departure 0450, watch sunrise on the bus, arrives around 0600 when the luggage storage facility will open. If you need to recheck your luggage, you can do so at HK station if you buy an airport express ticket (best price on klook.com)
If you don't have an octopus card you will need to get some cash from an ATM then buy something at 7-11 or one of the restaurants to get exact change for the bus.
Then go for a walk before it gets too hot.
First departure on the Airport Express train from Airport to the City is at 05:54.