I want to share a fantastic resource that I have been using for a few years. They are called Formative Assessment Lessons (FALs) that are developed by the MARS Shell Center. The lessons have detailed plans to explain how to use them in the classroom. There is minimal prep work and it gets the kids really thinking! It also gives the teacher a chance to see how the students are doing with a particular concept (hence the "formative" part of the name).
I tried one called Solving Linear Equations in One Variable in my classroom. The task asked students to classify different equations into 3 categories: Always True, Sometimes True, or Never True. Whoa, was a blown away at what my students did! Not that students didn't make mistakes-- I was just so impressed with how engaged everyone was. I walked by groups and heard great conversations and debates about how a equation should be solved.
These lessons are a great resource and I plan to use one per unit.