] Certainly A Midsummer Night's Dream has a play in it, just as Hamlet does, and The Tempest has the pagent that Prospero sets up for Miranda and Ferdinand, but are there others I'm forgetting?

Can't think of any off the top of my head.

] I've certainly read critics (the best of whom was Nigel Alexander) who take the view that the play is not just a means to prove whether to take action against Claudius, but mostly an action against him in and of itself.

"Capturing his conscience" Oooohh, I like that. Hadn't caught that one yet. Thanks!

] Are there other reasons why Hamlet has spends so much time on acting?

Well, in the bigger picture it think the presence of so much talk about acting and plays is because this play is all about appearances. Everyone is playing a role: Hamlet is pretending to be mad, Claudius is pretending to be innocent, Polonius is pretending to be an idiot and a good father, Laertes is pretending to be a virtuous brother, Ophelia is pretending to be a good little girl, Gertrude is pretending to be a virtuous wife and concerned mother, even R&G are pretending to be Hamlet's friends. I may be stretching with some of these, but I wonder if all of the characters who die are somehow pretending to be something they are not and those that are left standing (Fortinbras and Horatio) are just as we see them?