Passive Time-telling, Magnetic Watches and more
I think that being able to make a method, with which a person can tell the time passively i.e. without having to look at their watch/phone/sundail would allow us to investigate the limits of brain plasticity, circadian rhythms and also being pretty cool thing to do
My main ideas were to either use some sort of electromagnet to stimulate a magnet inside the body at the end of every hour. More esoterically you could stimulate it with a different frequency every hour (although I don't have an implant yet so I dunno how well you can perceive frequencies etc).
My other idea was to use a thermo-electric generator, to produce a shifting heat pattern with an "N" shaped wave - so it peaks at every hour then slowly decreases till the 1/2 hour mark etc, although more experimentation into how well the ambient heat from the peak dissapates and if the body can actually notice and make a "time map" of past peaks and stuff to actually give you a passive perception of time.
Both of those methods could be conceivably housed in a watch-sized assembly, probably even much smaller for the electromagnet