The Maple Leaf Lounge definitely. Do not use the Plaza Premium Lounge as it is just sub-par and crowded the last time I was there.
You definitely could go into town. The Canada Line is just across from the terminal as you exit, and it's pretty easy.
Not sure if you also have the same hours of connection on the way back, but if you do and you go back into security for a US-bound flight, bring your GE card if you have one, and that will get you into a different security line. I've never had to bring my GE card but for Canada I'd bring it.