Well, it is hard to say, Mandela did a significant expansion of the anti-government activities in the 1960s. As far as I know, he created a division dedicated to sabotaging. Besides that… I guess there wasn't much. He and his co-defendants were just acquitted of treason in 1961, and because of the fear of another arrest, they went underground for a while. Still, same year Mandela became a commander in chief of the guerrilla army. So, he learned how to fight, obtain weapons, etc. He became known as a Black Pimpernel. Still, it is hardly relative to anti-government activity expand. It was more like preparation or smth.
In 1962 he got arrested after his return to South Africa. By that time he was living underground for almost 17 months. He was accused of leaving the country and incitement to strike illegally. Thus he gets in jail for next five years. But there was another span of the story in 1963. A police raid found documents outlining the group’s plan for guerrilla warfare. Due to this fact, Mandela and another eight co-defendants were charged with sabotage and conspiracy to overthrow the government. The trial became known as a Rivonia trial and established Mandela’s central role in the whole issue. Thus, Mandela and seven others were convicted to life in prison. Mandela was sent to Robben Island prison where he would spend next 18 years.