Simulation and dissimulation are two excellent themes within this complex Shakespearean tragedy.
simulation: imitation or enactment, as of something anticipated or in testing; the act or process of pretending; feigning. (Random House Dictionary)
What is interesting about Hamlet is the way that drama itself is such a key theme; other characters simulating, or pretending to be, particular people. The Players are key characters, and Hamlet at one stage delivers lines specifically to them in order to give them instructions on how to simulate effectively:
Hamlet: Speak the speech, I pray you, as I pronounced it to you--trippingly on the tongue; but if you mouth it, as many of your players do, I had as lief the town-crier had spoke my lines.
The advice continues, and it is clear that Shakespeare here is parodying the way that certain actors attempted to simulate in his day. Simulation is thus a key theme of this text, and this is highlighted by the play-within-a-play and the various references to acting and drama.
However, whilst simulation is certainly a key theme, dissimulation, or the deliberate acting or misleading of others through one's actions without their awareness, is an even more prominent theme.
dissimulate: to disguise or conceal under a false appearance; dissemble; to conceal one's true motives, thoughts, etc., by some pretense. (Random House Dictionary)
Think of the numbers of characters who try to trick and deceive each other. Hamlet deliberately puts on an "antic disposition," Ophelia is sent by her father and Claudius to see if Hamlet is in love with him, Rosencrantz and Guildenstern are told to work out what the problem with Hamlet is, Reynaldo is sent to spy on Laertes.
And so the list continues. Characters are not honest with each other, for a variety of motives, and this is something that Hamlet in particular struggles with as he sees all of his friends, relatives and lovers (except for Horatio) try to deceive him to differing degrees. The audience of this play are plunged into a world where everybody has an agenda and is trying to somehow trick others, and it is very difficult to know, at times, where their real loyalties lie.