Ok, since people were asking about something actionable, here is the general approach that has worked with our first child, and hopefully will as well with the second (but with a completely different set of interests).
First, it hopefully goes without saying that whatever your child’s college “highlight reel” shows, it has to be your child’s interests and your child’s accomplishments. There is no point to pushing specific activities on your child unless there is a strong underlying interest. The best we can do as parents is help them shine in the interests they already have. In essence, I think of my role here as a supportive coach, guiding my children towards what will help them shine.
Step one with D was to assess what she both enjoyed doing and where she had real skill. I wrote in another thread that my D always loved art, but I didn’t really know where she stacked up relative to others at a national level. So in 9th grade, I suggested she attend National Portfolio Day with a collection of her best works. By the end of the day, we had highly positive reviews from multiple top art schools. RISD told her that she already had more than enough technical skill to thrive there, but only needed work on her art “voice”.
From this experience I knew that art would be a wonderful complement to her eventual college application, even through she had no plans to major in art. I encouraged her to spend more of her free time on art, and less on other activities (like martial arts). Which worked out great because every moment she spent on art was pure joy, whether it was creating it herself, or volunteering to teach it to others. She spent considerable time on both during her high school years, and it substantially lessened the other pressures she had.
She was also a major science kid in high school. She found a topic that she particularly loved during a summer at Hopkins CTY, and was able to leverage into doing three summers of research at a top medical school. I was the person encouraging her to apply to various places, but since we didn’t know anyone there, it was her talent that got her “hired” and called back year after year.
Her eventual Common App essay was a beautiful blend of her two main, but very different, interests. It was very much an essay unique to her, and her love for both came through. And these two interests were supported in other parts of the application as well (awards, etc.), and her art talent was validated via a submitted portfolio.