You could write about how Jack conceals (unsuccessfully) his affairs from Emily; about Robbie's concealment of his identity when he impersonates a higher-ranking officer in part 2; there's also that weird thing McEwan does at many points in the novel, when he shows a disembodied part of a thing, which, when unconcealed, turns out to be something else (i.e. in part 1 when Briony sees her mother's leg through the window, and it appears to float, before she sees it is her mother; Robbie also sees a leg in a tree in Part 2). That repeated image of concealment is often a reference to how characters (especially Briony) make bad conclusions based on a partially concealed view they have of a situation. Additionally, the ultimate concealment of the novel is arguably its writer: Briony writes the entire novel and conceals that fact until the end. She conceals her cowardice from the reader, as she reveals in London 1999 that she actually never went to see Robbie and Cecilia in their flat - they both died. We, as readers make a bad conclusion based on a partially concealed narrative.