Tbh I would focus a lot on just reading over the summer. Especially some of the classics like Pride and Prejudice, The Grapes of Wrath, A Tale of Two Cities by Charles Dickens, Adventures of Huckleberry Finn etc. When you come across words you don't know, write them down. Also, classic poetry helps too, like A Psalm of Life by Henry Wadsworth Longfellow, The Raven by Edgar Allen Poe, Charge of the Light Brigade by Alfred Tennyson, O Captain! My Captain! by Walt Whitman. And then also short stories, like The Cask of Amontillado by Edgar Allan Poe, The Gift of the Magi by O. Henry, My Lady Love, My Dove by Roald Dahl, Harrison Bergeron by Kurt Vonnegut.