Always provide shelter from bad weather with a clean place to lie down. Horses really do enjoy lying down, despite the myth that they only sleep standing up. If you don't have a barn, a three sided loafing shed can suffice as protection from the summer sun, flies, winter wind, and storms. This means scrub tubs and buckets regularly. Groom horse often to remove dirt and to stimulate the skin and coat.
Horses are happier if they have company.
If you own only one horse, perhaps a neighbor's horse can suffice. A miniature donkey, goat, or even a chicken can be a barnyard companion as well. Feed them at approximately the same times every day. If you must go away, employ the services of a dependable caretaker and leave a phone number for your veterinarian in case of an emergency. The veterinarian will ask you for these vital signs when you call in an emergency.