Agree with lots of transitions and cavelleti. And try things like rein back to trot, the rein back really engages them behind. Just a few steps of trot, while the quality is uphill. Also agree with Manni - you must have energy to create engagement, so if your horse is behind the leg, get her forward (not saying she IS behind the leg, just something to be aware of). In the canter, I LOVE leg yield - it really lifts the horse in front.
I also like trot leg yield to a canter transition. Leg yield gets the horse’s hind leg and hips working. Then the canter transition tells you whether the horse is truly in front of your leg - finish the LY, and promptly canter - if it takes a few strides, you have another issue to deal with.
Also agree with ExVet - if you have access to hills, AND your horse is good on the trail, that can help strengthen. Unfortunately, many don’t have that combination, so back to arena work ideas.