August Strindberg's A Dream Play is a great example of early expressionism in theater. It also exhibits some elements of surrealism.
The characters of A Dream Play fit perfectly into the mold of expressionism, as most expressionistic characters are not individuals but caricatures of a certain type or group of people. On her journey, Agnes meets a great number of generic characters with names like the Officer, the Lawyer, and the Poet.
Another element of expressionism that we see in A Dream Play is an episodic structure. Rather than building up conflict over the course of one main plot line, the story has us follow Agnes as she moves through a sequence of episodes where she has different interactions with humans in order to learn about their misery.
Seeing as it's titled A Dream Play, it makes sense that the play has a variety of striking, unsettling, or dreamlike settings, which is another element of expressionism in theater. For example, we quite literally start off the play in the clouds, where "the constellations of Leo, Virgo, and Libra are visible, and from their midst the planet Jupiter is shining with a strong light." At the end of the play, we have the juxtaposition of a nightmarish "wall of human faces, questioning, grieving, despairing" and a flaming castle with a huge chrysanthemum blooming on top of it.