In Stalking the Riemann Hypothesis, Rockmore discusses how Bernhard Riemann, as per custom, submitted three potential areas of research for his habilitation. Gauss was the chairman of the committee responsible for picking which of the three Riemann would work on, and, perhaps because of his interest in non-Euclidean geometry, he selected the third one, regarding the "foundations of geometry". Riemann's work there would, or course, lead to the development of Riemannian geometry.
What were Riemann's other two submissions? Did he go on later in life to research those topics?