No-stride jumps are a good exercises of gymnastic for the horse, it really helps the horse to be more supple. You can also work on transitions between the obstacles. For example, you jump the number 1, then trot 4 strides after, then jump the 2 and 3 in canter, trot before the 4... It is useful to work on the speed and control.
I am really sorry about the video quality but the exercises are interesting and helpful: https://www.youtube.com/watch?v=lTPT5uXZtjQ
You can work on impulsion, control, direction, balance and straightness. It also helps you to land on the correct leg after a fence.
I used to work with young horses. You can put some cavalettis in no-stride so your horse has to really rise his legs and it will make him developing a good reflex. Your can also work on the speed with transitions between the obstacles.
Square oxers, ground poles before fences, and gymnastics, but, depending on how young the horse is, these all need to be done lightly. They put a lot of strain on the hocks.