A：现在忙什么？ B：现在忙去玩。 As the Sentence A you can see "你在忙什么 or 你准备干什么 What are you buysing for or What are you going to do. I am not sure which is what your meaning with. As the sentence B you can answer with 我正在玩 or 我准备去玩。I am playing XX or I am preparing to xxxxxxx, in case,which is I am prepaing to play outside

A：在忙什么呢？ B：去玩儿！

准确来说不对，去玩并不是一件持续性的事情，甚至不具有实际的动作，你可以回答：我出去玩，或者忙着去某个地方比如，我忙着去学校，忙着走路，忙着打球，但不能说忙着去玩