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.

Your app writes 2 TB/month of logs that are queried heavily for 7 days, rarely after 30, and must be kept 7 years. Which S3 setup is most cost-effective?

S3 storage classes trade cheap storage for slow, sometimes paid retrieval. When you already know how access drops off with age, the cheapest setup is a lifecycle rule that walks the data down the tiers as it ages. A single class overpays, and Intelligent-Tiering solves a problem this question doesn't have.