Vocabulary ought not merely be associated with tedious assignments; Words are fascinating! I have grown in my vocabulary primarily by reading literature and listening to lectures, but I have also learned by playing with words: Scrabble, Crosswords, Jumble, Balderdash, and more.
When students complete their assigned work early (and well), I encourage them to put their minds to work on something constructive. One way to do this is to make use of some online word activities. Here are a few to get you started:
Mad Takes (like Mad Libs)