I think setting up active directory domain controller with all the DHCP/DNS and group policies is a number one thing to do, if you don't know how to do that.

Another thing would be running a Linux server and have a website. Learn how apache and Nginx works. And how to use them together.

It also helped to understand networking and virtual networking from non Cisco perspectives. I have a ccna and net+ and setting up opensense+pihole with network services was very weird, it felt completely different compared to ccna and net+ studies.

Well and of course having experience with virtualization. Learning different types of virtual storage and just in general how virtualization works.

The last thing is options but it is something that I decided to do, that can help you with networking(however there are other things you could set up that would be more useful). I would set up the gns3 server. This would help you with networking, especially if you are trying to study for network certs after ccna. But like I said, there are other projects that you can set up, that will be way more useful as a beginner.