I've taken a few trips out of the city. A few options for destinations:
-Elora: I haven't been here personally, but I've heard great things from friends about it. Mostly back roads once you get out of the city. It's a bit NW of Guelph
-Niagara Falls: a more popular destination. You can follow the Waterfront Trail (http://www.waterfronttrail.org/), which is pretty good. I believe you can also take your bike back on VIA rail if you don't want to bike back (it's about $30)
-Anywhere east along Lake Ontario: the eastward along the Waterfront Trail. You can head towards Kingston, but don't have to go quite that far. The trail alternates between good and kind of crappy.

In terms of actually getting out of the city, I'd recommend taking the GO Train out to its farthest extent in the direction you're going and heading from there. It's a good way to get a jumpstart on getting out of the city. I've taken the GO to Oshawa and then biked up to Peterborough (Trent University) or east along the lake. There are some nice trails that will get you partway out of the city, but nothing consistent, which is why I like GO.