I am not experienced with 'Dramatic Monolouges', but this question is similar to many others where the writer doesn't want to give "an obvious answer/fact" to the reader.
@Rasadashan gives some good suggestions, and I aggree with them, but without the story these suggestions will probabbly be hard to use/follow.
In all cases, I believe it depends on the story, the characters and the events that have taken place.
What your "character" includes in the monologue will or should be a reflection of the personality of the character and his/her connection to the other characters and the events that have taken place.
Creating the 'puzzle' for your readers to solve, should be based on what you'd like them to work on and what you feel confortable creating.
What kind of puzzle would you enjoy having to solve if you were reading/listening to the monologue?