Insurance policies claiming each other is a good one (if you have it, which we didn't.) This is specifically for common law, but works for marriage too, it's what I got in my additional doc request:
Client Information: Please submit documents proving your ongoing cohabitation with your sponsor. This may include but is not limited to: joint ownership of rental property, joint rental receipts, joint utilities accounts (electricity, gas, etc.), important documents that show the same address (e.g. identification documents, insurance policies, driver's licenses, etc.).