I recently started going to the library again (there is one across the street from my house!) and they actually have free passes that you can take to go to the AGO (among other places) so I picked one up and off we went!
The AGO is huge and there is something for everyone. It's nice that it has everything but at the same time, its a lot of walking so make sure you wear comfy shoes! I find the visitor's map a bit hard to follow so we never know if we're missing something. Also, because the collection is so big, everything starts to look the same in the end and then nothing really stood out. We checked out the Surreal Things exhibit and that was the best part of our visit. My boyfriend was fascinated by Thomson Collection of Ship Models. They were so intricate and incredibly well made.
The AGO has a great espresso bar upstairs with an amazing view of the city so make sure you stop by for some coffee or tea. I am in love with those floor to ceiling windows and shutters! On the street level, they have a fabulously chic restaurant and there is also cafeteria type place on the concourse level.
If you plan on making regular trips there, look into getting a membership. There are lots of perks and definitely worth the money.
Oh, and before you leave, don't forget to check out their gift shop! They have an amazing selection of prints, jewelry, house ware and souvenirs.