Here are some ideas Jessica :) http://cyrinecherif.com/basic-flatwork-you-can-do-with-your-horse/
The best way to muscle the entire body of your horse is to go on walks with difference in height. Then you can do a lot of dressage exercices such as lateral movements, closes transitions, half turn on the quarters...
Then you should ask your teacher for more details because every discipline has its own musculation program as we don't all use the same muscles.
I hope i could help you :)
If you can work with poles on the flat, I would suggest getting a series of trot and canter poles. Then you can have a full body workout and you can work on other things as well.
Lots of trotting and working the hind end
Engaging the hind end while trotting