Compile a list of 10 sentences with modal verbs in them (before class) and while you read out the sentences (in class) ask the students to use their imagination and write down who they think you could be talking to (anybody in the world that is)
You shouldn't wear that - I could be talking to Lady Gaga
You must train harder - a Footballer
You shouldn't smoke - My brother
You mustn't speak Spanish - Fernando in the class
You could try playing for Manchester Utd again - Ronaldo
and so on, getting feedback where necessary.