What's a fun piece of free software I should learn to use this week?

I need to quench my thirst for knowledge and learn something new just for the sake of it.

I've already learned:

• Equations and pivot tables in Excel

• Vector graphics in Inkscape

• Music mixing in rekordbox

• Personal VPN on a raspberry pi 4 with OpenVPN

• LAMP stack web hosting

• Streaming & video capture with OBS

• Manual & automatic backups with FreeFileSync

Things I'm open to:

• FOSS (even beta) or free-as-in-beer software

• A high or low learning curve

• Tools for niche fields that I'd otherwise have no reason to learn