Each time your class studies a new theme or topic, give them the assignment of coming up with questions and answers relevant to the topic that would be good for crossword puzzles. At the end of the unit, you can create a crossword puzzle based on the students' ideas. After creating the puzzle, let the class work on it. Eventually you will have a crossword puzzle for each theme you teach and you can have students answer them both before and after the unit. Using puzzlemaker.com makes it easy.