TodaysMeet makes it so easy to create a discussion board---you don't need an account to make one. You can also give it a unique name and set up when you want it to expire. Then just give your students the URL. It will prompt students to provide a "nickname" and voila!
Consider asking thought provoking question prompts or asking students to share ideas or questions on a topic. This is especially useful during a time that students may be watching a short video in class, as students can contribute thoughts, ideas, or questions they may have while the video is playing!
Oh---and if you do want extra features such as access control? Make a free teacher account on the website.