The Simics Trail, as it's known to mountain bikers (or Lake Laurentian Loop for the hikers), is a great, technical challenge for those that enjoy a ton of rock riding.
There are essentially three entrances to the trail system (Laurentian University behind the sports field, Lake Laurentian Nature Chalet, or from Ida Street) and as it's a loop, can be started at any entrance and can be ridden in either direction.
The description here starts at the Ida Street Connector
and circles the lake in a counterclockwise direction. The trails are very well-marked, and for the Simics trail, follow the red markers. There are also several maps at key intersections, so it's difficult to get lost. The trail can be broken down into two sections, the east and west. The east side is definitely the more difficult with significantly more rock riding.
From the Ida Street Connector
, head right at the first red marker. You'll quickly encounter a steep, loose rock climb, so make sure you're warmed up and brought a good dose of energy with you. As you head around the east side of the lake, you'll have several tricky climbs and descents over Canadian Shield rock. You'll be amazed at how your bike will stick to the black rock, so make sure to give them a try.
As you pass the Nature Chalet and start heading back towards where you started, you'll still encounter many rocky sections, but the most difficult sections are behind you. Along the west side, there are several opportunities to get a bit of flow and enjoy the scenery.