。
That girl absolutely loved climbing trees.
Literal
That girl [topic-は] tree-climbing [subj-が] very-loved.
木登り 'tree climbing' uses the nominalizer ~り on the masu-stem of 登る (登り) + 木 'tree' as a compound noun — the productive pattern for turning verbal activities into noun activities: 山登り 'mountain climbing,' 縄跳び 'rope jumping,' 虫取り 'bug catching.' 大好き 'to love very much' is an intensified 好き using 大 'big.' Textbook example of the [ACTIVITY]-noun + が + 好き pattern.