Other things that we included:
Government ID that had the same address on it for both of ours (drivers licenses, identity cards, voting registration).
We showed my work and permanent residency visa's in my partner country which were obtained due to the common-law status of our relationship.
We spent 6 months travelling together though Asia so we included all our entry and exit stamps (and visa's) as they are the same in both of our passports, as well as copies of all our flights to and from Canada together to visit my family.
Our region specific form also required 3 statutory declarations about our common-law relationship so we included one from my partners, one from my partners brother and one from a mutual friend, along with several other regular letters from friends and family.
There is a list of things to include in form IMM5490 as well as the region specific form. If you follow those you should be fine.