If you are planning on going to the Nanaimo area, it's worth the extra drive north to the Horne Lake Caverns (it's about 1 1/2 hours). They have quite a few options depending on how daring you are. We went last summer with 2 kids (8 & 10) and I'm not a thrill seeker at all and took the Family tour. You don't have to be super athletic, but probably should be comfortable in confined spaces. The best part is there are no creepy crawlers in the caverns. Bring your digital camera, you can take awesome pics in the caverns. Check out their website for more info.
http://www.hornelake.com/

We've been to the Island quite a few times, and there are lots of great things to see, but this was the highlight of our last trip. Afterwards we drove through Cathederal Grove and to Port Alberni. From there you can keep driving to Tofino (we didn't go this time). If you've never been to the West Coast, I would recommend going to Tofino/Long Beach as well, as it's quite different from the Vancouver shoreline.