I found federalism to be one of the most difficult concepts to teach in social sciences. After last year's election, I made a lesson that really gets at the Constitutional basis for federalism by closely analyzing the Constitution itself and the Tenth Amendment. You can modify this to be either a group activity, or teacher-led. The structured, repetitive reading of the text worked well for my students and gave them a deeper appreciation for the concept.
I tied it into the resistance to the Trump Administration's policies and a discussion of the Republican strategies against things like Obamacare.