What are three astonishing things Goodman Brown sees during the night?

When Goodman Brown reached the place of the midnight assembly, he found four pine trees ablaze and other vegetation burning. He saw a rock structure like a pulpit. Gathered around, singing an evil hymn, were many people he recognized and thought were righteous and holy. Many Christians who attended his church were there, including Deacon Gookin and his pastor. One woman looked like the Governor's wife, but Brown couldn't be quite sure. Along with people he had always thought were good people were those of poor reputation, people he knew were vile. The pure virgins associated with the criminals without showing any disgust for them.

When he came forward, he saw two other figures that appeared to be the ghosts of his dead father and mother. The one who looked like his father urged him onward while the one who looked like his mother tried to "warn him back." As he approached, two women brought in a veiled woman who was there as a new convert. The women were Goody Cloyse, who taught the catechism, and Martha Carrier, some type of witch. When the slender convert's identity was revealed, Brown saw that it was his own dear wife, Faith.

Of all the astonishing "things" Brown saw that night, perhaps the most astonishing were the ghosts of his father and mother and, most distressing, his wife being presented as a convert to darkness.

What experiences did Young Goodman Brown have in the forest?

In the forest primeval, Young Goodman Brown suffers a fall from innocence that is not unlike that of Adam.

When Brown ventures into the forest in order to test his faith, he loses this faith after witnessing the black mass and seeing his wife Faith there. As the pink ribbons of his once innocent wife "flutter lightly down through the air," Goodman cries, "My Faith is gone!" and he learns the terrible significance of this loss.

Goodman Brown experiences such a dramatic change in his character because he is initially so confident in his faith. But, this faith is challenged as one by one the seemingly saintly members of his community demonstrate that they are in league with the devil. For instance, when Brown happens upon Goody Cloyse, who has taught him his catechism and who he recognizes as "a very pious and exemplary dame," he is surprised to see her in the forest. Then, he is shocked when this "pious old lady" notices the traveler "put forth his staff and touch her withered neck with what seemed the serpent's tongue" and she cries out, "The devil!....Ah, forsooth, and is it your worship indeed?" Furthermore, she laughs and engages in conversation with this traveler who has ironically assumed the form of Brown's grandfather. 
Later, when Brown sees Deacon Gookin and Goodman Brown, he

...caught hold of a tree for support, being ready to sink down on the ground, faint and overburdened with the heavy sickness of his heart. He looked up to the sky, doubting whether there really was a heaven above him.

Of course, the most spiritually devastating sight for Brown is that of his wife Faith "trembling before that unhallowed altar" at the black mass in the forest. He calls to her, "Faith! Faith!...look up to heaven and resist the wicked one."
At this point Goodman Brown loses consciousness. When he does awaken, it is as a changed man. For, having confronted the universal nature of sinful man in the forest, Goodman Brown becomes cynical and misanthropic. Henceforth, "misery unutterable" pursues him as he has completely loss his religious faith and his belief in the goodness of humanity.

What experiences did Young Goodman Brown have in the forest?

Well, critics have suggested many different responses to this question throughout the years. However, it is important to identify the psychological nature of what Goodman Brown experiences. Here we have a young man whose name allegorically suggests a man who is trying to be good and be a Christian in a world where religion is very important. However, the events that he witnesses that one night in the woods shake his faith and understanding of Christianity to the very core, leaving him a dark, depressed and brooding man. Note the way that one by one the various saints of his village, who he is used to looking up to and respecting for their zealous religious practice, are exposed as shams and shown to be truly evil. Consider how Goody Cloyse is presented:

As he spoke he pointed his staff at a female figure on the path, in whom Goodman Brown recognised a very pious and exemplary dame, who had taught him his catechism in youth, and was still his moral and spiritual adviser, jointly with the minister and Deacon Gookin.

This spiritual giant in Goodman Brown's life is shown to talk of union with the devil and riding her broomstick, clearly indicating her evil nature, and pointing towards the lesson that Goodman Brown learns in the forest: no matter how impressive our outward spirituality and piety, within all of us there exists a propensity to evil that cannot be taken away. This is perhaps the truth that Goodman Brown experiences, and this is the truth that changes him so radically.

What event does Goodman Brown witness in the forest and who attends?

In Nathaniel Hawthorne's short story "Young Goodman Brown," the title character comes to witness a satanic mass deep in the forest outside his hometown of Salem. In attendance are all sorts of supposedly virtuous people as well as people who not seen as virtuous. Here are a few quotations taken from the story (I'm just listing without introducing or discussing them; don't do what I'm doing in a formal essay!):

the shape of his own dead father

a woman, with dim features of despair, threw out her hand to warn him back. Was it his mother?

the minister and good old Deacon Gookin

Goody Cloyse, that pious teacher of the catechism, and Martha Carrier, who had received the devil's promise to be queen of hell

It was strange to see that the good shrank not from the wicked, nor were the sinners abashed by the saints. Scattered also among their pale-faced enemies were the Indian priests, or powwows, who had often scared their native forest with more hideous incantations than any known to English witchcraft.

Most importantly to the story, of course, is that Young Goodman Brown's wife, Faith, is also present.

This is a very popular story. View the various links from the main link given below for more discussion of the story.