Cherry Pirate hasn't been heard from for a couple of years, but the thread is still largely correct. To clarify some logistics: Camping on Saltspring and at Galiano's Montague Harbour is somewhat limited and busy, could be an issue on weekends. Galiano's Dionisio is bike-accessible, great, a long way up the island, no services nearby and largely empty for political reasons (Google "dionisio access" for details). Saturna is perhaps the most bike-friendly especially for those uncomfortable with traffic on narrow roads, ample camping, fewest people and services. Don't overlook Gabriola just because it's not on the same ferry route, I've heard good things about it but have no personal knowledge.
As long as you are OK with hills any of the islands would be an excellent choice. Hills, hills, hills! The Gulf Islands are nothing but hills. Anyone who tries to soothe your hilliphobia is lying. Maybe with good intentions, but lying.