eager beaver

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun One that is exceptionally, often excessively industrious or zealous.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun idiomatic, colloquial One (especially a child) who is very excited or enthusiastic to begin a task.
• noun idiomatic, humorous A woman with a high sex drive

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an alert and energetic person