Heya.
I think its great to start small and create a log of all activities you have done big or small. Just because I know that when you obvi want create the best application possible and seeing everything you've already accomplished will give you a ' Hey, well done. You aren't at 0%' .
Now to actually answer your question 😭...
1. I think go as high as you can in piano, it's better to be top in a few things than ok in many. I think also because you started as self-taught the admissions officers will eat that up.
2. What are you genuinely interested in... Like you said you like art so why not do a research project surrounding art or maybe even creating a piece of art to submit.
3. Passion projects are very good also.
4. As for leisure, don't do it if you don't mean it. Admission officers can sense whether or not you are doing something for an extra point or if it's a genuine passion or interest. So try think of something out of the box like maybe a 10K charity walk to raise money for children who can't afford instruments.
5. Whatever you do make sure it has something to do with your passion. They want a well-rounded class not student, put all your eggs in one or two baskets ( maybe find a connection between the two) rather than one egg in piano, art, tennis, and I don't know 'science'.
AND BE YOURSELF!