I agree with the other posters that you should check with each college. Many colleges that do allow deferrals/gap years will require you to accept their offer of admission first and then request the deferral. In that deferral letter, you’ll have to spell out why you want the deferral and what you’ll do with the time. As noted, many colleges specifically say that you can NOT enroll in any college during that gap year.
Regarding merit scholarships, again check with each school. However, I’ve found in our research that many colleges will allow a deferral and you can keep the merit aid for when you start. Just note that the merit aid usually doesn’t change, so tuition/room & board will likely go up, but your merit may not.
Regarding things to do during the gap year, I’m trying to convince my own D26 to take a gap year and go somewhere and study a language for a year. I’m not sure I’ll convince her, but I think that’s a useful way to spend a gap year while also not being overly academic (learn the language without worry at all about the grade.) A medical deferral also sounds very reasonable, but just be careful about enrolling in college courses. Taking things for non-credit, I think, maybe allowed at some schools, but again you’ll have to check each one.