Qantas offers check-in at Hong Kong and Kowloon Airport Express Line Stations from 07:30, up to 90 minutes before departure. The train takes about 25 minutes to/from the airport, but add time for baggage reclaim and immigration.

I would say head to Hong Kong Station as then you'll be in the CBD. You might be able to dash to The Peak, but the queues for the tram can be long depending on the time of day. The Star Ferry is a short walk from the station. You could hop on that for photos of both sides of the harbour. Breakfast at Mandarin Oriental or The Peninsula (best to book for these). What do you want to do/see?