Kangxi did significant things in both the military and the cultural realms. His skill in these areas and his long reign combine to make him what some call the greatest emperor in Chinese history.
Militarily, Kangxi worked hard to expand the empire and also to ensure that it stayed together. Early in his reign, he had to suppress the Revolt of the Three Feudatories in which the lords of some provinces south of the Yangtze rebelled against the central government. Kangxi also defeated Mongol armies and Russian armies. It was also during his reign that Taiwan became a part of the empire.
But Kangxi was not just a military man. He was also very interested in intellectual things. One of his most important accomplishments in this area was his commissioning of the Kangxi Dictionary of over 47,000 Chinese characters then in use. He also commissioned a huge encyclopedia. These actions are significant in terms of the cultural and intellectual life of China.