Create a board game in which each square asks the players to perform some sort of task. For example, they might need to find the opposite of the word "cold" before they can progress to the next space. You could combine this game with other learning elements as well. For example, you could require students to find rhyming words or a word that starts or ends with a particular letter before moving on to the next space.
Create a crossword puzzle in which all of the answers are the opposite of the clue. You could also print one from the Boggles World ESL website. Depending upon the level of your class, adjust the difficulty of the words. For beginners, use simple selections such as "cold," "hot," "summer" and "winter." For the more advanced learners, have them find the opposites of more complex words, including "remember," "predator," "doctor" and "crooked."
For younger students, use flashcards to get them acquainted with the opposites. You could create flashcards that just have words on them or add pictures to stimulate visual learning. Divide the class into teams or pairs. Hold up the flashcards to each team or pair at a time, and see who is able to guess the most opposites in the shortest amount of time. Award a small prize, such as a no homework pass, to the team with the fastest time.
Bring students to the computer lab and allow them to work in pairs if there are not enough computers for each student. Allow them to play online games, such as Fun School's "Connect the Opposites" game. They will draw a virtual line from different opposite words, such as "day" and "night" or "happy" and "sad." Since the game is geared toward preschool students, assist them if they need help with reading the words.