Hi Noah,
You can take the express train from the airport Hong Kong Island or Kowloon( directly outside the Arrivals Hall) The express train will get you to Exchange Square/ Central Station downtown HK island in about 25 minutes, give or take. You can head to The Peak for a fantastic view but you might not have time for the tram so may want to consider a taxi ride. You could take the Star Ferry to Kowloon ( a short trip).
Day tours from the airport are readily acquired as well and might take a load off your mind regarding getting back on time. They're about HK $10-30 for 4, 6 and 8 hour trips and will pick you up at the airport and get you back. Can be useful if you're unfamiliar with the area and public transit. Tours cover temples, Aberdeen floating resto, old towns etc.
HK Airport ExpressHK WalksA Few Hours in Hong KongI'm not exactly sure what else you can do in a short time (maybe someone else has suggestions) but you may just want to take the express into downtown and take a look around. I haven't been in a long time and when we were there we had a week layover.
Getting through customs could take an hour or so, depending on how busy the airport is. It's hard to say. On the flip side, the airport is massive and many people stay and walk around on layovers although 8 hours might be a long time!