After being introduced to open-ended questions in math about two years ago, I was sold on how powerful they can be in the classroom, but it can be difficult to create high quality questions. Jo Boaler at Stanford has created an excellent site called "YouCubed" that has a wealth of activities for all ages that are engaging, open-ended, and for all levels.
You can search activities and tasks on the site in many ways (grade level, topic, etc.), but I especially love going to Grade -> "Low floor, high ceiling" tasks. These are perfect for engaging students of all levels in the same room. Go ahead, explore, and try some new things in the classroom!