Lots of good suggestions in this thread! A few additional ones that I don’t think I’ve seen yet:

• Testing/QA server (eg: test existing software’s major upgrades before upgrading your “production” environment, test new services without impacting your “production” environment, test new operating systems/virtualization software/etc.)
• Learn automation (eg: Terraform, Opentofu, Ansible, etc.) or horizontal scaling (eg: Docker Swarm, Kubernetes, etc.) to try improving future upgrades and/or high availability
• Media center PCs (eg: Kodi, LibreElec, OSMC, etc.) or gaming PCs for various TVs around your house to replace Apple TVs/Google TVs/etc. or gaming consoles
• Home Assistant