You could prepare a quiz ahead of time on the reading selection. Let the quiz focus on the pages to be read. Ask students to raise their hands when their reading is done, give them the quiz and watch. Since the quick readers are often gifted students, ask questions not at the knowledge level, but at higher level thinking. Ask inference questions too which everyone finds tough. Ask students to write not only the answer but the page and paragraph or line number which proves their answers. Collect and check the quizzes to know if the quick readers are skimming or truly gaining knowledge.
You could ask quick readers to outline the reading passage. If it is nonfiction, then the outline could name the way information is presented, such as chronological, problem and solution, cause and effect, or whatever is appropriate. Then the students could write one sentence per paragraph describing the information in each paragraph. If the reading selection is fiction, then the outline could state the type of writing, such as description, dialog, action, or whatever is appropriate. Writing one sentence per paragraph might not work for fiction, but one sentence per scene or character might. The point is for the students to prove to you that they comprehend what they have read.
You could ask students to choose five words from the passage that they don’t understand or that they think their classmates might not understand and use a classroom dictionary to look them up. Then students should write each word in sentences to show what the word means.
You could ask students to write one (or two or more) questions about the reading which require thoughtfulness to answer. Collect them, shuffle them, and then use them for class discussion or homework.
Notice that all of these assignments focus on the original reading selection and either extend or deepen students’ understanding of it. Students need only paper and pen and possibly a dictionary to do the work. If you have the extra assignments printed up to use as needed, you can pass the appropriate one out any time a student finishes early. And most of the ideas work well in science and social studies classes as well as in ELA classes.
Of course you could always have early finishers take out books and read them. If the books’ Lexile numbers fit the students’ reading levels, this works. But it does not enrich the reading lesson, and it could cause resentment among the slower readers who might feel punished for their slower progress.