Having recently returned to Linux after a twenty year hiatus, I’m currently enjoying OverTheWire as a good terminal refresher.
OverTheWire offers various wargames that let you practice and test your skills in security concepts. You can join the chat, visit the game pages, and connect to each game using SSH on different ports.
Good for you! I’ve been free of windows for a decade I think. There’s nothing I miss as most of the programs I like and need have linux versions. The main program holding me back was reaper, an audio editor, and when they released an alpha version for linux I erased my last windows hard drive. I only run debian stable, but I’m not too fussed with bells and whistles.