Check out What Do You Do When You Finish a Sprint Early? by Dr. Dan Rawsthorne. Here is an excerpt:
So, let me talk about the easy stuff first. One of the best things that can happen to a Scrum Team[1] is that it finishes its work early in a Sprint[2]. It amazes me that Teams are confused about what to do, but they are. So here goes… If the Team finishes early, it seems to me there are two…
The full article is available here.