。
She traveled around Europe.
Literal
[topic-は] Europe [path-を] traveled.
を旅行する uses the 'path-を' marking — を on a noun denoting an area or route across which movement happens. This isn't direct-object を but rather the path/movement-area marker, also seen with 歩く ('walk'), 通る ('pass through'), 渡る ('cross'). ヨーロッパ is a loanword borrowed via Portuguese in the 16th century — the 'Europa' form survives intact rather than being respelled phonetically as 'Yūroppu' might be more recently.