Hi Olivia,
I have done a home exchange to Vancouver Island (Campbell River - further north than Nanaimo) with our 2 kids when they were about 13 and 11. It was one of the best holidays we have had as a family. We were able to do some really interesting things with them. Whale watching is a graeat day out and very interesting. We also had a great trip we took to the north of the island - a place called Telegraph Cove. From here my son , daughter and dad when on a deep sea fishing trip, they caught 2 x 32lb salmon - which we were able to have canned and sent on to us in Ireland about 6 weeks later ! Stayed here in quaint fishermans houses in the harbour area.
Other great trip was to a place called Tofino - its on west side of island and has vast stretches of beach with huge waves - its wild and beautiful and the drive there is great.
You can also go canoeing which is big there.
Forrestry is huge there and in Campbell River there is a massive paper plant which we were able to tour and another day we took a trip up to top of mountains with a guide in a small group - where we learned all about forrestry and met some workers - we were above the cloud line.
We were there about 7/8 years ago, so don't know if its still the same.
We were there in early August and while the weather is nice, its not too hot and you can have some days at the beach.
Hope this helps - let me know if you want to know anything else ?