eager beaver

(noun) a person who is very enthusiastic and works very hard

Example Sentences:

Roberto was an eager beaver when he started work on his first job.

I was a real eager beaver during my first year of university. By my third year, I was bored.

From the Blogs:

Nanny State Eager Beaver Busybodies

Media:

Similar idioms:

1. busser
2. bottomless pit