Choose a paragraph from a textbook or another passage and ask your students to use a thesaurus to completely rewrite it. Instruct them to change as many words as they can. When they have finished the assignment, ask them to read their newly written paragraphs out loud and decide whether the original or the new version sounds better.
Give your students a worksheet with an unfinished paragraph on it. Leave blank lines throughout the passage with the words that would typically be used in parenthesis next to the space. Assign the students to use a thesaurus to find a more interesting, yet relevant, synonym to write in the blank.
Although a student might be able to find a number of different synonyms for a word in a thesaurus, it does not mean that the word will fit into the sentence properly. Teach your students how to find the best synonym for the sentence by assigning them to identify which words they think best fit into a phrase and which do not sound right. Choose a few words and ask them to identify the best synonym, a couple of good choices and those words that do not actually fit into the sentence at all. You can gauge their level of understanding of using a thesaurus from this exercise so you know if further instruction will be necessary.
Turn learning to use a thesaurus into a fun game for your students. Divide them up into teams and show them a series of pictures. The team that can use its thesaurus to come up with the most synonyms for the object in the picture the fastest will win a point. The team with the most points at the end of the game will win the thesaurus game.