For the third year running, Apple is holding an Apple Watch Activity Challenge to help celebrate National Parks. Last year's workout was designed to mark the 100th anniversary of Grand Canyon National Park in Arizona. This year, users are asked to earn awards on August 30th by hiking, walking, riding, or running a mile or more. The activity can be logged by any app that adds workouts to Health.
Activity Challenges give Apple Watch owners the opportunity to collect trophies in the Achievements section of the iPhone Activity app. Successfully completing a monthly challenge workout can also net the participant exclusive stickers to use with FaceTime and Messages.
As Apple states in its support document about Activity Awards on Apple Watch:
When available, you can view these special challenges in the Activity app on your Apple Watch or your iPhone. These challenges are only available for a limited time, and if you don't earn the award during the specified period, it will no longer be available.
Check out the stickers that can be earned by completing the August 30th workout this year: