This Airport is far away but seven hours is enough time to see some of Hong Kong.
Take the
Airport Express from the Airport to Hong Kong Station. Trains comes once every 12 minutes and arrives at Hong Kong Station within 30 minutes (They claim 24).
When you arrive at Hong Kong station, you are directly underneath the current tallest building in Hong Kong. The IFC (This was the building under construction where Angelina Jolie flew off in the second Tomb Raider). From there, you are within walking distance of Central Pier and Hong Kong Legistature/Parliament.
Across the Harbour is the history clock tower in Tsim Sha Tsui, that can be a Ferry ride away on the
Star Ferry.
I am not going to suggest you go up to Victoria Peak, I imagine it would take a bit of time to get up there on the Peak Tram and on the bus.
That's all I can think of for now.