Okay well gymnastics and grid work are always good jumping exercises. They help with timing and strength. Lots of pole work and hill work are always great as well! And don't underestimate plain old flatwork either, the better your flatwork is, the better your jumping will be (you can do some dressage exercises to help your horse get his/her rhythm down and build muscle )
If you are looking for some exercises to do while you're not riding to get stronger, start with some squats and crunches. You can also do planks and wall sits. If you stand on a step and stretch your heels down, it will help transfer to your heel position while you ride too :)
Gymnastics are good for your horses muscle and agility
Exercises fro you to do to strengthen your muscles or exercises you can do with your horse while you ride?