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The Sony Wena 3
It attaches to an existing watch face (Sony doesn't mention how), and its three-line screen can communicate with your iOS or Android phone via Bluetooth. It can display notifications, measure your steps and heart rate, connect to Siri or Alexa, and has built-in NFC for mobile payment systems. The battery takes 1.5 hours to charge, and is said to last for about a week... More here.
Sony keeps banging the smart buckle drum and version 3 does look like an improvement. I'm not convinced, however, that it is small enough to fit in.