Ah. This building is so amazing. Granted, I'm not the biggest fan of post-modern esque architecture (blame the bricks and cobblestones of Boston for that) but the AGO is truly a cool building. There's the crazy windy staircase in the middle that always makes me want to run through it. But perhaps the best feature of the building is the front room that is pretty much floor to ceiling windows that go up at least 20-30 feet. Great place to have a sit and pretend to ponder while soaking up some window filtered sun. Also, the backside of the building's stairwells have some great views of the city/CN tower. Unfortunately, the building's cool post-modern style means that it's a pain to navigate. Expect to hit some dead ends and turn around.
They also have a great collection. Not a huge fan of their permanent on the first few floors (oy, more group of 7?) but the top two of contemporary is always pretty fun. Worth your time to check it out. Free Wednesday nights (630-8 I believe) but call/check ahead. Went for free last time as well as they were admitting anyone under 25 in. Another plus is that it's insanely centrally located. Don't have to trek far before after to whatever you may want to do next/a TTC station.
AGO you kick the ROMs butt.