Question: What kind of activities can teachers propose in order to improve their students' English vocabulary ? 

Answer: Reading will definitely help, especially if it is from a variety of writers, genres and time periods.  Word of the day type things work for some people, but others totally blow it off.  I had a teacher once who gave us extra credit for finding Vocabulary/ Word of the Day words in our books.  You could do something similar for students who take the time to learn a random word from the dictionary well enough to use it, or teach it to the class.
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