Give students a bunch of words or phrases to ‘sort’ any way they like. Students may choose to categorise the words by meanings, sounds, grammatical properties, or something completely different.
You could then get feedback as a class, or simply monitor each group and see how they sort the items. The fun part is asking them repeatedly to sort the items in different ways.
It may be interesting to throw in a few words that may be unknown, to see how students cope with the unknown words.
You may even want to try this for introducing unknown words, where students don’t have the meanings of the words beforehand, and clues could be given for subsequent sorts – “these words are all connected with food” for example.