If you have photos taken together at home,with both family members, travel and tour/vacation photos, copies of travel tickets, hotel reservation receipts, online booking details,e-mails, chat records, phone bills if any showing both your numbers etc can be additional proof of relation.
if you guys have joint bank account, and if you have same address you can provide bills,same home ownership, your proof of purchase with both your names on it,letter from friends supporting your relationship..anf if you have insurance where your partner or spouse is the beneficiary..that will do
You will find sections 5.25 and 5.26 of the OP2 manual helpful. It describes the characteristics of "conjugal" relationships (spouses, common law partners, etc) Read those sections and try to provide evidence for each of the types of interdependencies. It also lists examples of evidence most of which rouvie and wilson have already mentioned.
Photo's of you as a couple in several stages of your relationship are good. Photo's taken of you together at tourist attractions, if you have them. Photos of your partner with your family/friends, photos of you with your partner's family/friends.
I even included an email exchange with my partner's parents, discussing an upcoming visit and my partner's grandma's health.