Somewhat halfway between practical use and just messing around for fun.
Several years ago I built a GPS NTP clock out of an RPi3 and an Adafruit GPS hat. Once I had the PPS driver installed, it’s precision/drift got pretty good. According to its own self measurements, I got pretty dang close to NIST stratum 1 NTP servers, but those are hundreds of miles away so that measurement isn’t super precise. It’s still running today, clocking nearly 24/7 operation since (checks shopping history) 2017, though I replaced the breadboard and mini module with a full sized hat with the same chipset in 2021.
Recently I acquired a proper hardware GPS clock and I stacked the two against each other and found out my RPi did not half bad and can get between 0.5-10ms of the professionals (literally I’m pretty sure I’d need more precise measuring equipment to tell the difference between the two at this point than a regular computer). Now my homelab has fully redundant internet-disconneted stratum 1 time. Been half considering if I could write a GPSD driver for it as a joke, but I know upstream won’t accept it because it doesn’t offer SOOO many features they’d need.
As for what else - I just kind of keep an eye out for projects related to GPS and high precision time, like the open source atomic PCI card that was released a few years ago. Finding out what people are doing to get better and better time is just downright interesting.
Outside of the time world, it’s just fun to see what projects people come up with relating to maps and navigation. Stretch goal once I have enough server horsepower is to make a render-capable Open Street Map server with my home region loaded to start with, but eventually I’d like to get it to the point where I can load and process world.osm. That… Requires a LOT of CPU and SSD space.