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I'm looking forward to going to the zoo.
Literal
I [topic-は] zoo [to-へ] go [nominalizer-の]-[object-を] looking-forward [adverbial-に] waiting-[state-ている]-[explanation-んだ].
楽しみに待つ ('wait eagerly, look forward to,' literally 'wait in the manner of pleasure'). 楽しみ ('enjoyment, something to look forward to') + に (adverbial) + 待つ ('wait'). A set expression for anticipation. 動物園 ('zoo,' literally 'animal-garden').