Well this place is like a second home to me. It can be a bit of a walk to your Gate so I would pay attention at Check In as to where it is before you go chill out at one of the two Lounges. Air Canada Lounge or if you have an AMEX Platinum Card, you can hang there.
Lots of food options here. And also other shopping options to.
If you are flying out early in the morning, you may need a Hotel for a night, there is a Sheraton Gateway Hotel attached to Terminal 3 that is very nice.
Lots of Rental car outlets available. Oh and of course lots of Tim Hortons Coffee/Doughnut Kiosks..