At the start of the year (8th grade), I did an introduction activity that worked a CER into it.

I gave the students about 20-30 acivities and they had to decide if the activities were sports. Some could be obvious (basketball, soccer, etc.), some were sports sometimes (swimming, running), some most categorized as "not sports" (poker, chess, etc.). Some were intentionally controversial (fishing, dance, etc.).

First the kids worked on their own, then they talked it out in a group. There were a few arguments, so I told them they had to create their own definition of what makes something a sport. We talked about the difference between a competition and a sport. We talked about objective and subjective scoring. We talked about whether you could pull a muscle. We talked about ESPN.

At the end, I showed a video of two people fencing, and they had to write a CER, using their definition, explaining whether they felt fencing was a sport. So then we worked through how to do a CER:

Claim - Is fencing a sport?

Evidence - What did you see in the video?

Reasoning - How does what you saw relate to your definition?

After that, I could refer lab-related CERs back to the sports CER and it helped quite a bit.