My young horse starts to discover the lunge, but I never know what to do, do you have ideas of exercises?

You can also work with driving reins, it allows you to do more exercises than the lunge and you also use your voice, which is interesting for work later.
Lauren is right, do a lot of transitions so he can get used to your voice. If you want to diversify the exercises, you can put some ground poles!
You can do easy things if the horse is young (downward and upward transitions) and really work with your voice so you set up vocal codes, it will be helpful for later!
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