Kidport
Kidport: Antonyms
This site has a five question quiz from Kidport that offers immediate feedback on each question. Elementary pairs of words like cloudy and sunny, good and bad.
Quia
Quia: Awesome Antonyms Match Up
Match up pairs of antonyms using a concentration, simple matching or flashcard approach. Site claims that over 90 pairs are used, so multiple plays are possible.