Cost

How do you cut EC2 costs for workloads that sit idle nights and weekends, without relying on someone remembering to turn things off?

Automate the stop and start instead of asking anyone to remember it. Scheduled Auto Scaling actions or AWS Instance Scheduler both work; the choice mostly comes down to whether the fleet is already behind an Auto Scaling group. Either way, tag-based scoping is what keeps it from ever touching something that shouldn't be stopped.