The Opposites is a simple, yet great app to have on your iPad. This app allows students to match opposites. Did I mention it's free?
To start off, the students select "play" and the level. The higher the level, the harder the words. According to iTunes, the harder words include words from biology, poetry, medicine, and Greek prefixes.
The students will see words float to the top of the screen. Once they find a match, they click on each word. This will turn them a different color and then pop if the match is correct.
Students can also select the dictionary and review the words from each level. I wouldn't consider this cheating at all. After (or before) completing a level, students could select a match and create a visual for what the words mean and how they are different.