Having lost my fort in a huge riot of unhappy dwarves I have been thinking of ways to make them happier. Also making dwarves who are resting stop going insane. So far I have two different units for entertainers and both could be used.
1. Musicians: They play music in the meeting halls and provide a happy thought over an area to anyone who listens to it. Better instruments and skill make more happiness. However, if they play badly they can make an unhappy thought over the area because bad music just makes people mad.
2. Jesters: They provide single targets with multiple happy thoughts using a variety of jokes and gags. They can be assigned to people or they can just wander around entertaining random people. Perhaps some nobles can demand them as part of their retinue or in one of their rooms. If they're assigned to an injured person they make them happy all the time which keeps them from going nuts.
3. Actors: They could give plays which give a large happiness boost with a long duration but take a lot of preperation and setup time.
Another idea is that using the same joke or song over and over reduces the amount of happiness it produces. Making more songs or jokes/gags/props etc could be part of an entertainment workshop. Perhaps the construction of musical instruments could be moved here as well.
These unit types would make it easier to keep large populations happy and keep them from going crazy and murdering everything. Probably it would work to have a new Entertainer profession with skills Music, Acting, Jocularity, Playwriting, Songwriting and some kind of joke making skill. Also maybe include some specific skills for making musical instruments and setting up stages/costumes for plays.