Different terrain for horses such as access to sloping or hilly areas, and the introduction of obstacles such as felled logs and uneven surfaces could make use of different muscle groups.
Access to woodland and rough grazing, a sand-filled area or pond could also provide enrichment within the field or track setting, whilst also encouraging movement. Scratching posts would probably also be well received by horses, especially if access to suitable substitutes like trees is limited.
When available to them horses will naturally browse on (hedgerow) plants such as cow parsley, nettles, etc, as well as overhanging tree branches and shrubs, and access to these could be beneficial to your horse, as well as natural enrichment. If available this would typically make up around 10% of their diet, but can be as high as 50% in some cases. The greater diversity of plants available will allow your horse access to, and the ability to select from, the many constituents these plants offer, and the amount your horse requires to maintain their health and well-being.
Below are some plants that horses could browse on and benefit their health and well-being, as well as provide natural shelter:
Herbal supplements are sometimes added to an animal's feed however given the choice they may not choose all the constituents of these, but they could be broken down at a Herbal Choices session and offered individually; allowing your animal to choose. However, supplements prescribed by your vet should be continued and reviewed by them.