How does juxtaposition work in As You Like It? I understand juxtaposition occurs when two scenes are placed side-to=side but have contrasting effects. The only example I can think of is the descriptions of the court and the country. I need to think of a scenic juxtaposition in the play and how it works in terms of idea and/or characterization or theme.

Juxtaposition also comes into play during the famous seven ages of man speech given by the melancholy clown Jacques in act 2, scene 7. Here, juxtaposition is used to raise questions regarding the themes of aging, youth, and old age. Both Jacques and the old servant, Adam, present two different visions of the passage of time and its effects upon individuals.
In the speech itself, Jacques presents a nihilistic view of human existence as an empty cycle of foolish behavior and suffering. People are foolish and helpless as children, and then they return to being so when they become old. The very comparing of human life to a stage also trivializes all history and human activity, almost calling it no more substantial than fiction.
And yet, this somber, cynical speech is followed by the entrance of Orlando with his aged servant Adam upon his back. Earlier in act 2, scene 7, Orlando praises Adam's courage, loyalty, and "pure love" for him to Duke Senior. His words are in every way oppositional to Jacques' perception of old age as the domain of weakness and indignity. Adam is feeble, true, but his inner worth remains despite his lack of physical prowess or energy.
The juxtaposition of these two characters allows the audience to ruminate on these ideas about aging and time for themselves. In the end, Adam disappears from the play unexpectedly, not appearing during the final wedding scene, which might support Jacques' take on old age as having no stake in the happiness of the young.

The literary device of “juxtaposition” can be used in a variety of ways. It means to put two things side by side. The range extends from single words through the type that interests you, scenes, and can apply to ideas or characters. Although As You Like It is a comedy, it has serious elements to the plot, involving the conflicts between the two sets of brothers. In both cases, one brother has treated the other one unfairly, either depriving him of his inheritance or taking his rightful realm. One could juxtapose a scene in which Oliver and Orlando come into conflict, as occurs early in the play, with a different, contrasting scene involving the two dukes, who finally resolve their differences.
Another fruitful area to look for clearly contrasts is in the many instances of lovers courtship and wooing. One of the play’s key themes is love, and throughout the audiences is treated to many types of confused interactions stemming from conflicting ideas about love as well as the misunderstandings engendered by the women’s disguises. Two love scenes could be juxtaposed. The disguised character of Rosalind/Ganymede links these contrasting types of courting with Orland and Audrey respectively, or one could juxtapose either with Touchstone’s courtship of Audrey.

The main "scenic contrast" in the play is, as you say, between the court and the Forest of Arden. In this formulation, the court is a place of plotting and revenge, while the Forest is a place of harmony. We can contrast, in this light, the characters of Duke Frederick and Duke Senior: Frederick is self-centered and banishes Orlando, while Senior is congenial and welcomes newcomers. The idea is that in the Forest, things are real and genuine, but the court, with its ambition and paranoia, is somehow fake and dishonest.
Arden deserves a more careful look, however. Perhaps another pair of scenes to consider are in Act II: scenes four and five. In scene four, Touchstone, Rosalind and Cecelia have arrived in disguise at Arden, where they overhear the shepard Silvius proclaim his love for Phebe. Although Rosalind is moved, Touchstone makes fun of Silvius. Touchstone does not like Arden: it is a primitive place, without any of the comforts of court, and the people in it (if Silvius is any indication) are not very sophisticated. As he says, "When I was at home, I was in a better place."
In scene five, we get a different view of Arden. In this scene, Ainiens, one of Senior's courtiers, sings a song in praise of the pastoral life and peaceful nature of the forest. As with Touchstone and Silvius in scene four, here Jacques debunks this praise, making up an extra verse for the song that says that anyone who leaves everything to come to the forest "here shall see gross fools as he." In both scenes, the positive aspects of the forest and pastoral life are called into question by skeptical characters.