I have multiple things I tested and learned.
Firstly an opnSense firewall.
An Active Directory (aka setting up a Domain Controller with DNS etc) with a test client, DHCP failover (active passive)
When you have an AD (=Active Directory) you should try to set up an Exchange server, making mail flow rules etc. maybe a cluster
Docker
Reverse Proxy
And last but not least setting up Vlans -> I have a basic understanding and know how it works (and should be set up) but sadly haven’t actually configured it here at home
And virtualizing things and get some hands-on experience with VMware/Hyper-V/Proxmox/QEMU etc.
That are the things I have learned and improved my skills with at home -> At work we have no opnSense firewalls, but for learning setting up pfSense, opnSense or using an old firewall (regardless of the manufacturer) helps understanding access rules, NAT PAT etc.
Many 1,2TB HGST 2,5" 10k RPM SAS drives For backup some WDs out of external cases