I enjoy writing and am interested in furthering this as an extracurricular. What are some activities (ECs) or competitions in writing that carry great importance in college admissions (specifically HYPSM type schools)? Are there any specific magazines/camps/competitions that I should try to enter or write for?
For HYPSM, either a Pulitzer or Nobel would likely make them notice.
You have multiple postings asking if a bunch of different things (summer activities, GENIUS Olympiad, Yale Young Global Scholar) would be prestigious or help you to get into top colleges. You need to take a step back and understand that the elite schools have more exceedingly well qualified applicants who have done a wide array of impressive things than there is space to accept. So your best course of action is to stop looking for some kind of a silver bullet and instead seek out outlets to do things you care about and let your passion shine through.
And while @Eeyore123 answer was a bit tongue-in-cheek – the poster is on point. Short of some tremendous writing accomplishment (ex. a bestselling novel, publication in an elite journal) admissions officer will probably have seen it all. So if you want to pursue writing that is wonderful but do it because you want to – not because winning a particular contest or going to a particular camp will have “great importance” in the admission process. If there was some activity or summer program that would truly give an applicant a true edge in elite college admissions someone would be bottling and selling it.
Academics (including GPA,course rigor, standardized tests) is the most important part of the application. You are a sophomore and probably just have one year of a GPA and no standardized testing. So for now I would focus your time and energy on being the best “you” you can be in every aspect of your life (academic, social, EC, family life etc.) and stay off of CC until you are a junior in HS.