I'm probably among the best to comment on your situtation, since my girlfriend lives just outside of Toronto, and I have spent the last 15 months flying back and forth visiting her and her family. We often have to venture downtown on couple excursions to have fun, so I think I can help you out.
First, of course, do go and see the CN Tower. It's a remarkable sturcture, and it affords a remarkable view from up top. You can get there by taking the TTC Subway to Union Station and walking via the Skywalk. The Tower's ticket booth is located between it and SkyDome, the tall, white-roofed structure to its side. Tickets are somewhat expensive for what you get (just a view), but it's not too bad, especially with the ~65% exchange rate. I'd recommend to not get the Total Tower Experience Package, simply because it seems like a waste of money--all you get is two simulator rides and a movie down at the base. That stuff's been there since the tower opened, I've been told, and it's not all that thrilling. I would recommend that you get the next cheapest package, including the Look Out, the Glass Floor, and the Sky Pod. The cost should be around $20-25 CDN, if I remember correctly.
Be forwarned, though, that the lines around the CN Tower can get ungodly long at peak times (ie, on weekends and holidays). We happened to go up on Canada Day after a Jays game, and we spent more than an hour in line to get up to the main level, then another hour and a half in line for the elevator up to the SkyPod (the highest observation deck), then another 15-20 minutes waiting to get down. It shouldn't be that bad usually, but you'll probably have to wait, especially if it's a nice day.
Moving on, if you and your girlfriend are sports fans, you might want to take in a game at SkyDome, right next door to the CN Tower. The Blue Jays and the Argonauts play baseball and football there, respectively. The Jays games are decently attended and have an okay major-league atmosphere; the Argos are generally poorly attended, with an atmosphere resembling minor league or arena football. But good seats are generally very plentiful at either team's games, so if they're in town, you might want to check them out. Prices are not too bad for an evening of entertainment--we spent about $35 CDN apiece for the Jays (lower tier, 6 rows from the field), and $33 CDN apiece for the Argos (sideline, 15 rows from the field).
If you're into shopping, you might want to check out Eaton Centre, Toronto's main shopping mall, located primarily below the surface of the city. You can get there by taking the TTC to the Dundas station and walking through the underground tunnel into the mall. It's no Mall of America, but it's still pretty darn nice.
Another nice place to go for an afternoon is Centre Island, an island park out away from the Toronto shoreline. There, you'll find an abundance of trails, grassy picnic lawns, and beaches. There's an interesting hedge maze in there on the southern side of the island, which is very nice for a couple to get lost in (beware the gnat swarms on hot days, though!). Also, and best of all for me and my girlfriend, there's Centreville, a small amusement park on the island. It's targeted more towards kids than anyone else, but it's fun and cute to have a go at the Centre Island Monster, the ferris wheel, the carousel, the sky ride, and so on and so forth. The miniature golf course is also pretty fun, although you can forget actually playing well there--you'd stand a better chance on Pebble Beach. Plus, it's cheaper than Paramount Canada's Wonderland, the local amusement park--only about $20 CDN apiece for an all-day ride pass.
To get there, just head to the Harbourfront from Union Station (you have to walk underneath the highway overpass to get there--fortunately, there are sidewalks). Then pay the fare to take the ferry across to the island ($5 CDN, or $3 if you have a student ID). Pack a picnic lunch and you can make a fun afternoon outing out of it!
Some other recommendations I would make would be the Hockey Hall of Fame, and the Toronto Zoo (located outside the downtown area) and Niagara Falls (about a 2-3 hour drive away). If you need any directions or further assistance with any of these recommendations, please feel free to e-mail me!