Do a search on GTAM for Nippissing Rd and Commanda and you should get a few matches. The nippissing trail south of Commanda is apparently not passable by motorbike, but it is passable as you get closer to Magnetawan. South of magnetawan it's quite nice. There is also the Ahmic trail, Orange Valley Rd and Broadbent road for nice gravel. Look at the Bunny trail, Hurdville road and McDougall Rd as other great roads in the area. The 520 is nice as well. Check out screaming heads in Midlothian and the east side of the 518 and chetwynd Rd. If you like forest roads, then near Kearney you can also access the Deadhorse trail which is just on the west side of Algonquin.
That's all I got. I usually just zoom in on a map of the area and go exploring.