Raingear is a must for most areas. Don't get a poncho that flaps in the wind and scares the horses. Or if you have a good sailors two piece rain suit be sure to bring that along. Again, make sure if you have little ones that there rain gear fits them well over layers and is practical. Fly spray and suntan lotion.
Flies are common in most horse country.
Particulary for the back of your neck at the edge between the ends of your sleeves and wrists. Watch the perfumes and aftershaves you use. Certain ones attract insects. Allergy's to pollen, etc are usually less troublesome in the outdoors. But everyone is different, so be sure to bring what you need.