As busy as a bee is an example of:

Oxymoron: a figure of speech in which apparently contradictory terms appear in conjunction (e.g. faith unfaithful kept him falsely true )

Metaphor: a figure of speech in which a word or phrase is applied to an object or action to which it is not literally applicable.

Adage: a proverb or short statement expressing a general truth.

Simile: a figure of speech involving the comparison of one thing with another thing of a different kind, used to make a description more emphatic or vivid (e.g. as brave as a lion ).

“as busy as a bee” is a simile because there is usage of 'as' in the sentence.