Nanaimo is one of the best places to SCUBA dive on the West Coast.  Various navy ships have been sunk there as artificial reefs.  A dry suit is (almost) a must, as the water is quite cold, but the visibility is generally decent, and the wrecks have lots of nooks and crannies to explore!  There are quite a few places that will take you out to the wrecks or to other spots, such as Ocean Explorers Diving.  A two-tank dive will run you $120. 

A fun thing to do in Nanaimo is to book a private kayaking tour.  The Nanaimo Tourism web site lists several local kayak outfitters, including Adventuress, which specializes in kayaking for women.

Newcastle Island is very beautiful and worth exploring if you have additional time.  There is a short ferry that you can take to get there, or go with a kayaking outfitter. (If you kayak on your own, please be aware of the seaplane runway there!)