Duty: Shakespeare expands on this concept concerning an individual's responsibility or motivation to feel compelled by various circumstances to act in a certain way.
Action vs. Inaction: Shakespeare, in my opinion, perfectly conveys how fearful feelings and situations define our ability to act or not act.
Spying/Distrust: In general, Hamlet develops this theme very well, emphasizing how distrust and espionage can be detrimental to healthy coexistence.
Appearance vs. Reality: Shakespeare uses this concept throughout the play to emphasize how appearances are frequently different from each character's reality.
Mortality: Through Hamlet's concerns and the final scene, Shakespeare affirms the inevitability of death and the natural course of life.