Did you take any pics when you first met and spent time together? Photos are the best evidence. Take pics when you get married, and try to include other people there as well - friends and family. Photos with each other's family, friends, coworkers etc. are also good. CIC wants to see that you two have a relationship that is well known to each other's families and friends.
We also sent screenshots of photos posted on Facebook, check-ins at places where we went together, status updates, relationship status updates (engagement announcement, marriage) etc. The screenshots included comments and likes from many people. This was done to demonstrate what I just said above.
We also sent testimonial letters written by people in our lives who had met our significant other. For example, my parents, his sister, his friends, my friends, his god mother, etc. They each wrote a short letter about our relationship, when they met our significant other, and what they think about us. The letters were not notarized, but they contained contact info.
He also gave me a joint credit card, and also added me to his life insurance, and we sent proof of that.
We sent a photo, appraisal and receipt for my engagement ring.
We sent copies of receipts of places we went to together, hotels we stayed in, our wedding rings, plane tickets visiting each other, passport stamps, etc. etc. etc.
We also took photos of wedding gifts we had received, cards of congratulations, etc.
We also each wrote up a personal statement explaining everything - why we decided to be in a relationship, why we decided to get married (so quickly), why we decided to live in Canada, why we only had a civil marriage, how we were planning a wedding reception the following summer, what our life plans are, etc. etc. etc. The application forms are pretty cut and dry, so this is your chance to put a human face to your story. Don't write "we clicked, and it just all happened." CIC isn't prepared to just take your word for it; it's their job to scrutinize, so you will have to describe your thought process.
We did sent excerpts from our whatsapp conversations, fb chat, skype call logs, copies of cards and letters we had sent to each other, etc., but when we got all our evidence back, they didn't seem to have really gone through it - they came back in the original clips. They probably just saw they were included, and ticked a box. The photos and facebook screenshots seemed like they had been looked at the most.