Where are the API playground and changelog in the docs?

The API playground lives in the reference section of the docs, under the Health and Fitness API reference pages.

The API playground lives in the reference section of the docs, under the Health and Fitness API reference pages. If a link moved after a reorganisation, start from the reference index.

API release notes are published in the changelog section of the documentation site, which lists product and data-model changes as they ship.