Say a word and have your child return its opposite: hot — cold, big — small, up — down.
When an easy chain is running, throw in something harder: gentle, empty, early. Let them think; the pause is where the work is.
Then swap and let them serve. The words they choose reveal which pairs are solid and which are still guesswork.
Opposites force a word to be defined by what it is not, which means holding a whole dimension in mind rather than a single label. Words stored in contrasting pairs are retrieved faster and used more precisely than words learned alone.