We really didn't send a lot, and it was one of my (many!) sources of stress throughout the process. However, we are American and Canadian, have no complicating factors, lived together for nearly a year before getting married (he lived and worked in my state for four years), and had no red flags, and our proof of relationship was obviously deemed adequate, as I landed back in October.
I put together a 4-page document. In the document, I had 8 colour photos showing the two of us together, including one from our very small, informal civil ceremony when we were married. I attached three trip itineraries, two showing us traveling together, and one showing me going to visit him where he was deployed. Three of the photos were from the trips represented by the itineraries I included. I wrote a couple of sentences or a short paragraph describing each photo. ("Here we are on our first vacation, Mexico, November 2011, getting ready to go parasailing. It was the first time for both of us. We stayed at X resort in Riviera Maya, but this was an excursion to Cancun. Itinerary attached for trip, Nov 17-23, 2011.") The last paragraph was just a declaration that (basically) I had a full and enjoyable life in my home state, and that the best evidence I could think of to show our relationship was genuine was that I decided to leave my family and friends and my secure life in my own country and follow my love to a region of his country where they don't speak my language (Québec) and where I can't work for the foreseeable future, just to stay with my love.
It wasn't a lot, and when I found out that many applicants had included the equivalent of small novels, I was stressed! However, as I stated, it was adequate for CIC.
I think it's important to try to look at your relationship objectively and be honest about any "red flags" you have. We had none. If you dated for a very short time before marriage, or if you met online and never (or rarely) spent time together in person before the wedding, if your family and friends don't know about your relationship, if you've had multiple short marriages and divorces previously, you need to think about the suspicions CIC may have about your relationship and include more proof to satisfy them that the red flag doesn't mean your relationship isn't genuine.