What did contact between the nubians and the egyptians mainly center on

Nubia was an Egyptian vassal for much of the ancient three kingdom periods. Though, Nubia would rebel during times of Egyptian governmental unrest. While trade would be the correct summary, what really was happening was that Egypt was extracting its tribute/tax from their Nubian subjects. This consisted mostly of gold (which was abundant in Nubia) and also Nubian/Kushite slaves who were used in the Egyptian army and desert quarries. 

A once impressive garrison fortress of the Egyptians, called Buhen, was built at the key location of the 2nd cataract (now flooded beneath Lake Nasser) where Egyptian forces could monitor Nubian resource exchanges into Egypt and guard against rebellion. Other garrisons like nearby Semna and Kumma served the same purpose.