。
She went on a picnic.
Literal
She [topic-は] picnic [to-に] went.
に here marks the destination/purpose of motion — when paired with verbs like 行く, 来る, 出かける, the に-marked noun can be either a place ('to the park') or an activity-purpose ('to go for a picnic'). ピクニック is an English borrowing that has settled comfortably into Japanese and overlaps with native expressions like 遠足 ('a school outing, day trip') or お弁当を持って出かける ('go out with a packed lunch'); ピクニック leans toward a leisurely, often Western-coded outing.