This idea was developed by the in-house design group of teachers at our school. It's a game played by teachers at the school who might be interested in collaboration but struggle to get out of their classrooms.
Ask all/any interested teachers across the faculty to submit their names to the group. Then, during a faculty meeting, put all the names of the interested teachers in a hat. Teachers are selected from the hat in pairs: each pair is expected to collaborate on a lesson of some kind before the next faculty meeting. Pairs will likely occur across grade levels and subjects. At the following meeting, the two teachers present on their process, including details about the way they collaborated and how the lesson turned out. Celebrating the risk-taking is essential.