One aspect of musicianship that a lot of people can feel uncertain about teaching is Improvisation. As you're just making stuff up on the spot and there are no right or wrong answers, how do you teach it? How do you assess it?! Where do you begin?!?!
A common misconception is that improvisation is just for jazz musicians and involves complex virtuosic solos only for the most confident musician. This couldn't be further from the case - in fact, improvisation should begin with your prep class.
I like to start with a spoken improvisation - making up a text to a steady beat. Once my students are able to consciously keep a steady beat then I do this activity with them, taken from a workshop with the wonderful Kate Thompson.
We all sit in a circle, with a blue cloth laid out in the middle. I explain that this is our ocean - albeit a very small ocean! - and that we are going to put some sea creatures in the ocean.
Out of my magic bag I pull several bath toys, squishies and models of various sea creatures, each time saying "I can see a ...." I start simple, stating " I can see a fish", then gradually add more detail, perhaps describing the colour or the texture of the creature ("I can see a turtle with big eyes").
Once all the creatures are out, I ask the student to choose a creature and we go around the circle. Each student has 4 beats to introduce their creature. Once again, we start simply - I can see a jellyfish etc. After a practice round, we make it a game: each student can only use 4 beats, if they take too long or don't speak in time with the beat, they are out.
In subsequent lessons, the students are given 8 beats and must describe a certain detail about the creature eg. I can see a jellyfish with wobbly legs (ti-ti ti-ti ti-ti ti-ti taa taa taa saa). You can complicate the game further by not allowing anyone to repeat an answer that's been said, or not describe the same creature feature (eg. if the person before you described the colour, you have to talk about how many legs, or how it feels).
Once your students are older and more musically literate, they can begin to transcribe their improvised patterns, or you can choose 4 students to improvise and have everyone else complete a rhythmic dictation of their pattern!
I also love tying in a book to these activities, so at the end of the session, I pull out Christian Riese Lassen's sublime "Secrets of the Sea" and sing it to the class (borrowing the melody from "Away From The Roll Of The Sea" below). This never fails to bring a class into a state of silent wonderment and is a lovely way to finish off a class!