The AGO always has interesting special exhibits - I just enjoyed the Ai Weiwei exhibit, and marvelled at the installation of his pieces.
The 4th and 5th floors are usual stops, as they are always a-changing, and then I unfailingly pause in the Kurelek gallery (contains my favourite "Don Valley on a Grey Day" piece) before ending my visit with a coffee at the 2nd floor cafe, under the "ribs" of the exterior Gehry structure. Even when the gallery is full and bustling, you can find quiet peace in pockets.
The restaurant FRANK is also awesome. They have the best brunch meals and excellent service.
I recommend the AGO for tourists and Toronto residents alike.