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How fun it is to play baseball!
Literal
Baseball [obj-を] do-[nom-のは] how fun thing [probably-だろう].
なんて~だろう is the set exclamatory pattern meaning 'how X!' or 'what a X!' — なんて adds astonishment/admiration, and the final だろう rhetorically marks the utterance as an exclamation rather than a question. More emotional than the bare 楽しい.