Interactive
Quia

Quia: All About Antonyms

For Students 3rd - 5th Standards
Matching, concentration and flash cards to help with understanding antonyms. Word search is also an option, but doesn't appear to connect with antonyms. Words appropriate to upper elementary.
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Kidport

Kidport: Antonyms

For Students 2nd - 4th Standards
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.
Interactive
Quia

Quia: Awesome Antonyms Match Up

For Students 2nd - 5th Standards
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.