A good way of generating essay ideas for any text is to consider one of its themes and to explore the presentation of that theme in the text as a whole. This can give you a specific focus for a very good essay. One of the themes that is presented to us in this novel is that of technology and the way that it breaks into the lives of the characters in this novel.
For example, Severo del Valle believes that the 20th century can be described as being about "light, science and technology." Esteban Trueba is a character who is mesmerised by technology and he seeks to exploit it to try and make life better. However, interestingly, in spite of the immense promise and faith that characters seem to place in technology, science does not yield the kind of answers and results that they expect it to. It is not able to prevent Esteban from shrinking or to heal Clara from her silence.
The novel therefore seems to point out the limits of technology and science. Old Pedro Garcia is a character who is used to do this. He is able to achieve feats that modern science is baffled by, such as dispelling the ant plague and setting the broken bones of Esteban. There also seems to be an implicit link between technology or science and magic throughout the novel. Note what Clara says about this connection as she speaks to Nicolas:
If you can't understand how the telephone works, how do you expect to understand miracles?
Thinking about the presentation of science and technology and its relationship with magic in this novel would be the excellent basis for an essay. Otherwise have a look at the other themes in the enotes study guide. I have included the link to this below. Good luck!