I keep records of alk, Ca, Mg and pH tests and how much of whatever I dose. All are handwritten in either a binder or coil-bound notebook. Can't be bothered wasting my time to transpose the written records onto my computer.
No need to keep track of maintenance because we basically do the same maintenance weekly.
As for new fish or corals, I only keep records of fish disease treatment, though I can't remember the last time I treated a sick fish.
I often make a video of new a fish, with the date of the video in the file name.
All light bulbs (or lightbulbs, if you prefer
) are changed every 9 months, and we mark that on the calendar in advance so we know when to change them next. Same with various RO/DI cartridge changes.