Trotting poles are great for building the topline, if you have no access to hills. Also, long and low work, where applicable. I like doing lots of lateral work while riding long and low in my warm-ups. If your just getting your horse back into work, then I would take it very slow. Lots of walk work, laterals, etc.
Backing is also a good exercise, but If I'm being honest, It has never been my favourite to go to.