Oh man, you've got lots of great options! I lived a Younge & St. Clair for a few months, and enjoyed the riding in the area. The whole Don Valley trail system is your playground.
In general, grab a copy of the Toronto Cycling Network Map. The numbered routes are generally really pleasant. Use those to connect to trail systems like the Don Valley, Humber Valley (though that might push you over 30km), Kay Gardner Beltline, West Toronto Railpath and the Waterfront trails.
I've attached a couple of specific rides I did (zipped GPX files). I hope that helps.