agree with all the others posts and a local YYZ resident. no point in staying at the airport for lounges. KLM's sucks and BA's is only open for a little time while you are there.
CX flies 2 daily flights to HKG from YYZ. yours is the 01:30 so you really do not need to be back at the airport until 12. so lots of time to go into YYZ. probably only 15-20 minutes going back that late so you can leave around 11:45pm to return...
if cost is not an issue, just take an airport limo. $56CAD each way to the Royal York hotel...from there you can walk into the PATH underground - unless on Sat/Sun since they stores are closed - and make your way underground to the Eaton Centre. for some shopping. heck, if the leafs/raptors are playing, pick up some tickets. games start at 7 and are done by 9:30-10. plenty of time to get to the airport.
you could even walk around Queen Street East - although could be cold.
for a nice dinner, if you have not been to YYZ, make a reservation to dine at the bar at Canoe -
http://www.oliverbonacini.com/Canoe.aspx
not cheap but great views. PM me and I will put you in touch with my buddy who is a manager there and they will treat you very well...in my former life i was a fat cat that did way too many lunches at that place.
any other questions about the city, let us all know. lots on this board from YYZ.