Here’s one idea:
http://www.algonquinpark.on.ca/visit/recreational_activites/minnesing-mountain-bike-trail.php
There are more glamourous destinations, but it is a challenging ride. It is an abandoned forest access road (50 years ago?) across some challenging terrain.
Camping is at the south end.
Consider avoiding the spring and early summer, as it can be very wet.
Also, if you have spare tires and tubes, it might be a good idea to bring them along.