Headspace has some lovely invisible instructional design. I've used it for years, so I don't need/want prompts anymore during meditation—what's the point? Each unguided day has a 30–60s pre/postamble with a small new idea to consider. Not part of the explicit value prop; no UI.
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The ideas are subtle, so one per day is an appropriate rate. And they're integrated into the medium and context of practice, fuzzing the boundaries between (their) idea and (my) practice. Imagine a big "tips!" article with all this content: I'd read it once and forget most of it!
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Reminds me of old "tip of the day" UIs, but much less self-important. When I use Headspace, I'm actively trying to build a skill, so I'm very interested in new ideas to bring to my practice; when I use e.g. Eclipse, I'm trying to accomplish a task, and the tips are in my way.