Activities for August usually have to do with the harvest -- harvesting the wheat, binding the sheaves, and threshing. Swimming and falconry are other possibilities. The zodiacal sign for August is the Virgin, Virgo.
Here are the August illustrations from four different Books of Hours:
August, c. 1405, from Les Belles Heures du Duc de Berry, illuminated by the Limbourg brothers; August, c. 1412, from the Tres Riches Heures, by the Limbourg brothers; August, c. 1500, from the Hours of Henry VIII, illuminated by Jean Poyer; August, c. 1515, from the Da Costa Hours, illuminated by Simon Bening.