One of the courses in my design education was literally just for making posters (one every week). We'd typically have a focus or subject, and then a few guidelines to steer the project.
It took some time, but after a while, I really enjoyed the task of creating "one-off" designs. Each poster is a fresh start with a different audience. I actually got to produce them as well (via screen printing or as large laser prints).
Typography // Illustration // Conceptual