Not stressful. The airport train is totally reliable and integrated into the terminal; just make sure you are back at the airport 2 hours prior to your departure. Allowing for an hour through immigration (won't take that long), you'll have about 4-5 hours at liberty; 30 mins each way into town, so 3-4 hours to see stuff. Worth doing!!
I would get off the airport train at Central; walk round down-town a bit (no need for anything to be open) - you might get as far as Hong Kong Park. Then to the Star Ferry Terminal, catch the ferry across to Kowloon. Walk east along the foreshore where the elevated viewing platform is, as far as you want to, then loop back west to Nathan road; and further across to the Ocean terminal / Harbour City malls which should be open by that time. Large, and aircon!!
If the weather is hot, then you might just want to inhabit the many shopping malls.
A classic thing to do is the Peak Tram to one of the lookouts behind the city, but on a tight time frame that may be stressful and not worth it.
Lots more to see, but that's just the skeleton. Unfortunately its tricky to get onto the Airport Line from downtown Kowloon - Kowloon station for the airport is a bit to the west. May just jump in a cab (good and metered).