There are three main mountains plus smaller areas in the Cowichan Valley with trails. Mt. Prevost, Mt. Tzouhalem, Maple Mountain and the lesser Mt Sicker and Mt Richards all of which have trails in various states of repair. Duncan is the biggest town in the North Cowichan, 50km south of Nanaimo on Hwy1. If you are to stay there, the coastal communities of Maple Bay or Cowichan Bay have a bit more of touristic ambience. 

Some of these trails are signed and shown in the Mountain Bike Vancouver Island (4th edition) guidebook.

Cowichan, near Duncan BC area is worth a visit by bikers because of its unique proximity to the ocean and the mellowness of its weather; which lends itself to a long riding season. The mountains where the trails are built are round and relatively low (Maple Mountain, Tzouhalem, Prevost, Sicker, Brenton all rise a mere 500 or so meters above sea level) . The terrain has some unique rock features and dirt composition similar to the coast. BUT it sees a fraction of the rain normally falling on the west coast of the Sea to Sky area or Vancouver Island. It also sees very little snow creating a year round riding destination.

To this good weather, add some pretty special bio-geo-climatic features like red-barked arbutus trees, HUGE stands of Garry Oak, sword ferns so green they defy photoshop saturation, moss so thick the beds deform under footstep, pine needle carpets and thick loam. Add a community that actually supports bike trails and hasn't made an effort to put golf courses or condo tracts through perfectly good green space; then mix in lots of amenities like good food & accommodations. Suddenly the Cowichan looks like a good place to ride bikes to people from Victoria, the Lower Mainland or from other places.

Riding Areas:
Maple Mountain - Now home to the first sanctioned mountain biking trail in the Cowichan Valley, Maple Syrup, by the Cowichan Trail Stewardship Society. Overview of area here
Mt. Tzouhalem - Vast convoluted network of trails. Overview map is here. Cool ride starts at the top and ends in Genoa Bay which has a great cafe for lunch. 
Mt. Prevost - Mostly downhill trails. Map

A Typical Weekend can be seen here

Hotels and B&B's from Tripadvisor. The B&B's offer places in nicer areas and a more home feel. The current top Trip Advisor pick is Funky Frog B&B

Food from Trip Advisor in the Duncan area. 

Food specific to Cowichan Bay which is a cool ocean side destination.