As of my last knowledge update in September 2021, Apple does not offer a standalone activity tracking device that you can keep in your pocket like a small dedicated tracker. Apple's primary focus for activity tracking has been through the Apple Watch and the Health app on iPhones.
However, there are other fitness tracker brands on the market that offer pocket-sized activity trackers designed for your specific needs. Some of these devices are smaller and more discreet compared to a full-sized smartwatch or phone. They can track steps, distance, calories burned, and some basic fitness metrics. Here are a few examples:
1. Fitbit Zip: The Fitbit Zip is a small clip-on activity tracker that you can attach to your clothing or carry in your pocket. It tracks steps, distance, and calories burned.
2. Withings Pulse: The Withings Pulse is another compact activity tracker that can be worn on your belt, placed in your pocket, or worn on your wrist. It tracks steps, elevation climbed, distance, and sleep.
3. Garmin Vivofit 4: Garmin offers the Vivofit 4, a basic fitness tracker that's small and lightweight. It tracks steps, distance, calories, and sleep.
4. Xiaomi Mi Band: Xiaomi's Mi Band series includes affordable fitness trackers that can be worn on the wrist or clipped to clothing. They track steps, sleep, heart rate (in some models), and more.
Remember that device offerings can change over time, and new products might have been released since my last update. I recommend checking the official websites of fitness tracker manufacturers or visiting electronics and sporting goods stores to explore the most current options available.