As with most things...Variety is the spice of life...
A combination of exercises is always advisable for improving fitness, condition and strength.
Here is a list of different forms of exercise to try with your horse:-
Polework - Begin with ground poles but once your horse gets the idea, raised poles are more effective at achieving increased range of motion in joints as well as improving proprioception
Hillwork - increases cardiovascular fitness as well as utilising a range of different muscles
TTeam exercises - I may advise certain TTeam exercises to help your horse develop proprioceptive skills and improve symmetry
Different terrains - Sand/soft ground, long grass (to increase step height) hard ground, rough uneven ground (balance and posture)