It's inevitable. Students will complete assignments and work ahead of schedule, while others are still struggling to complete an assignment. This is one of the "laws of education." Even when we ask students to complete work for another class or read a book, what do they choose to do? Some choose to use this time ineffectively and inefficiently. Why not use this time as a time for enrichment?
If you are lucky enough to have a few extra computers in your classroom, you may want to use www.vocabulary.com. Your students are given the challenge of answering a variety of multiple choice questions about vocabulary. The questions appear on a note card, giving it a classroom feel. The site has built a database of over 40,000 words and you have options for narrowing your choices down or getting clues. As you compete, you can accumulate points, making it a competition.
If you create a username, you will be able to track your progress and see how your vocabulary develops. What a great way to practice for those darn standardized tests, while your students are waiting to progress.