Everyone pretty much already mentioned the things that came to mind for me first.
I also liked the little things they did like opening cabinet doors and pulling out the stuff they used. Stepping over lights in the yard. Looking at the camera and making faces or shrugging at you when you told them to do something they didn't really want to do. When you told them to pose for pictures, their eyes would follow your camera movements (creepy and funny at the same time). They cuddled and spooned in their sleep. I even miss some of their idle actions- tapping their feet, rocking on their heels and clapping their hands together, making faces and raising their eyebrows, doing hand motions that reflected their strong interests.
