Last week, I set a goal to include one creative activity (composing or improvising) in every lesson – 22 total.
Here's a simple activity I used with my older students:
Play two notes (I usually start on Middle C, then choose an interval that's accessible for the student). Name the first note.
Have your student play both notes, then add a third. Then, it's your turn again.
Play all three notes, then add a fourth.
Continue taking turns, playing the melody and adding one new note each time until you come to a natural resolution (or you forget what you created!). (source)