I think you are going to struggle with answering this question without narrowing it down further. It is vital to remember that this work is both long and complex, and therefore it contains many different themes and elements, each of which that could be analysed and discussed. As a teacher, I would strongly recommend that you break down this question to focus on a particular theme within the rich tapestry of stories that we are presented with in this collection of tales.
One such topic that you might like to focus on is deception. Many of the tales focus on the way in which we can deceive others and what happens when we do so. The tales offer many different contexts for deception, whether it be for "good" motives such as Arcite in "The Knight's Tale" disguising himself to see Emily, his beloved, or bad motives, such as fox in "The Nun's Priest's Tale," who deceives Chanticleer in order to receive a tasty supper. However, apart from simplistic good/bad motivations, you also might like to think about the way in which deception is carried out for a didactic purpose, or in order to teach somebody something. We can point to the "old woman" in "The Wife of Bath's Tale," as she assumes this appearance only to educate the young knight about women and how they are treated.
Narrowing down your question will give you much more focus and allow you to explore a theme in depth rather than give superficial coverage of a range of themes. I have included some links below to give you more information about this important text which I hope will be helpful. Good luck!