I was born and raised in Duncan, and fished the Cow growing up before heading out to Calgary along time ago. There are some good runs above Skutz Falls (make sure you have a map as the Falls are wicked and you would have to get out before then. There is a nice run from Lake Cowichan to Skutz that has lots of nice runs but you really need a boat as access is little to none. Below Skutz there are lots of areas where you can do some walk and wading, or float. Be aware of high water as the river can be dangerous at that time. I did a float a couple years ago with Kenzies Fishing out of Cowichan Lake and we floated the section above Skutz and did quite well. Lots of browns! You can also check with Buckys Sporting Goods in Duncan (if there still there). They would have tons of advice for the Cow for you as well. Be ready to fish in the rain, and be wintered up as it can be cold, especially when floating.
Say hi to one of my favorite rivers for me while you are there, it is a great place to spend time on the water!
Cheers