Once worn mostly by dads on a serious schedule and the occasional Al Qaida terrorists, the Casio F-91W is a piece of 90s nostalgia that watch enthusiasts still tinker with today.
YouTuber N-O-D-E, for example, beefed up an old Casio F-91W watch with a few modern features. He removed the iconic face design and replaced it with a simple letterbox, revealing only the digital time and a sliver of the inner electronics. He replaced the classic green LED with a brighter white light, and stuck a ntag213 NFC chip in there for communication between the watch and a tablet. He also stuck an SD card slot on the back for more storage.
These simple watches beg to be modded: Watch Week forums user hansp inverted the colors on the screen on a F-91W with a simple linear polarizing film. Others have adjusted the LED light—which is notoriously shitty in the original, with the bulb shining at a 90 degree angle away from the stuff you actually want illuminated on the face—and added different colors or brightnesses.
Then there's the hydro mod, which involves pouring oil into the watch electronics. This, according to The Watch Site forums user Homocaballus, keeps out water if the watch is submerged. Olive oil gives it a greenish tint, while mineral oil stays clear. YouTuber Artur Martins hydro-modded his F-91W with the latter, and he claims it's survived snorkeling trips and swimming pools for the last three years.
Oddly, however, this might be the only piece of cult-status electronics left that hasn't been modded to run Doom. Yet.