I LOVE these Arduinos! I’ve learned SO much from them! …so many projects… Some of my favourites have been
controller for my reef tank --monitored and controlled temperature(relay board and temp probe), monitored PH(ph probe), ATO (automatic top off, float switches and peristaltic pumps), dosing with peristaltic pumps, controlled via webpage run on arduino, tracking and uploading all information to Sparkfun data server (I’d use Grafana now adays)
simple boat autopilot using heavy duty hobby servo and compass to point a trolling motor
MINIMOSD – on screen display for the video feed from my old drone. it overlayed data from the flight controller (altitude/airpseed/battery, etc) much like the new DJI drones do, this was way before that though.
SatNOGS Open source satellite tracker – Azimuth elevation tracker for antenna to track objects across the sky as they travel through space to ensure the strongest signal possible. This was the project that introduced me to 3d printing and led me to the MPCNC eventually. I connect it to software on the computer called Gpredict which lets me track basically anything.
MPCNC – obviously! 
I think it’s pretty awesome that a huge chunk of the 3D printer market is powered by them as well, makes things so much more approachable!