The Lower Don trail is fun and has some interesting branching paths; runs from almost the Lakeshore up to Sunnybrook park. Also, veering off to the east of it is Taylor Creek park.
For road riding I like to get out really early on weekend mornings and go around the zoo - not much traffic and some good rolling hills.
Anyone looking for a real challenge, or to get in some hard training: I recommend going down - and UP - Brimley Road south of Kingston Road about 5 or 6 times in a row.
Just don't try it on the weekend.
Rollerblades and families on the trail will drive you nuts.
There's some psychological thing that goes on, and people forget that their mountain bikes, hybrids and even road bikes can go off the paved path and ride on the grass to go around people, without shaking apart.