我觉得此句中理解为today修饰menu较为准确。副词修饰名词在英语中是常见的。如:
1. He thinks pop music today is as exciting as ever.
2. Several weeks ago, a young man moved in the room upstairs.
当然你的句子如果把today 理解为修饰整个句子,在一定语境也是可以的,相当于 Today, what's on the menu? 本句中不能理解为单独修饰is。
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