SysAdmin challenges/exercices

Hi there, I'm a SysAdmin newbie. I made a website last year and learned how to host it and manage it, while learning about an entirely new OS.

I do enjoy system administration and I would like to learn a lot more about it! As I learn best by doing, instead of reading or watching, I was wondering if you could suggest me a small list of all kind of SysAdmin challenges sorted by difficulty. I need ideas of new things to try and experience. For example, I thought about setting up my own mail server, etc.

What else could I do? I'm eager to learn! :)

Thanks in advance!