In Night by Elie Wiesel, what were the only things in which Eliezer took an interest?
I think that the answer depends on which particular portions of the book one is addressing. At the start of the book, Eliezer took an interest in religious studies. He sought the spiritual instruction offered by Moshe the Beadle and truly possessed a desire to want to further his connection to the religious aspect of his being through deeper study. Eliezer seems to possess some level of interest in the role his father plays in the community as both economic leader and political leader, but the opening section of the book shows that Eliezer is more interested in religious studies and the implications that arises from them.
As Eliezer's life in the camp progresses, he seems interested in survival. Eliezer's predicament demonstrates how survival became the most important element in being under the cruel control of the Nazis. Eliezer saw the bonds cut off that linked him with other elements of his being. He saw the connection to his community repudiated with the selections, as well as the connection to his mother and sisters severed with the movement of one line or the other at the discretion of Dr. Mengele. As the narrative continues, he does show an interest in keeping connection with his father, but as the time in the camps become more difficult and more excruciating, this becomes secondary to his interest in survival. Along these lines, Eliezer develops an interest in the elements of survival, such as food, getting another ration, or ensuring that threats to his being are neutralized. Continuing this to the end of the narrative, Eliezer's only interest is survival and ensuring that he lives.