While the applications will typically not be out until sometime in August of your senior year, many of the schools do not change their essays year to year, and most schools will have a “Why Us” essay. So you can get a good head start on writing.
Independent reading is great for so many things, but we rarely have time for this during the school year with all of our required reading. Some of the colleges actually ask you what books you have read in the past couple of years for your own interest. And the UK particularly likes to know what reading has influenced your intended area of study.
While the summers by no means need to be all work and no play, they do represent relatively more free time in your otherwise hectic lives, which can be gainfully used. But you should also have some fun! You will have a hectic year in front of you, and you deserve some down time.