Take a Break Button
The main button opens a random calm experience. Choices on the site include slow koi, drifting clouds, ripples, and a breathing-circle timer. The site describes these as short browser breaks, with no login and no autoplay sound.
How to keep it bounded: choose one scene for the online part of your plan. Leave when your time ends, even if there are more scenes to explore. You can also skip the online part entirely.