Open-source smartwatch that is fully arduino programmable (run by an ESP32), usually lasts 5-7 days and costs around $50.

Quite a few watchface examples exist online. From what I understand, the watchface is the firmware for this clock,
so no too easy changes there.

There are assembly kits with a case that can be bought for some $ more that will keep the watch a bit more safe (though not e.g. swimmingpool ready). The watch/display so far only exists in square shape.