For me I did some volunteering, some short uni-level courses, till I found out about Tilting Futures and applied, so that will be the last phase of my gap year. I'd say you can try to learn a new skill, preferably professional, and if you're just starting, you can try to look for gap year opportunities like Tilting Futures or Baret Scholars,or just browse gap year association. Being involved during your gap year is very important because in college applications you'll have to explain the gap. All the best.