I've cycle toured on all of the ferry accessible Southern gulf islands and the San Juan islands. I would re-iterate that they are hilly. Expect to spend half your time huffing slowl up hills, and half your time zooming at full speed down the other side. Having said that, I think they are still the best option for cycle touring close to Vancouver. The islands are great places to explore at a laid back pace, in which case there is no need to rush up those hills. Just get into your granny ring and plod along slowly. The traffic tends to correlate with ferry times. Sometimes it's best to just wait it out - that's certainly the case when disembarking from a ferry. Let all the cars go past, before tackling the inevitable first hill. My favourite island is Saturna, because it has very few cars, and some really incredibly beautiful parks. you can camp at Narvaez Bay. The ride up to the far end of Galiano is great too (and you can access Dionisio Point marine park by bike btw - you could camp there). I didn't have any issues on Pender, but I was there in the low season. Other campsites I'd recommend are Montague Harbor on Galiano and Ruckle Provincial Park on Saltspring. Prior Centennial on Pender is nice and central and in a pleasant forested park, but is inland. I don't know if you're including Gabriola, but it's worth a look as well!
The San Juans have a different feel to them, being in the States, but do have a lot to offer. I can give more info on my time there if you like.
I highly recommend checking out the cycle touring sections of BC Car Free. All online, and with lots of info (but some of it is out of date).