Hey Mag.
Try to find something else that needs to be done, especially if it's in line with what you've already completed. Try to figure out if there is a better way to do what they just had you do. Look for a part of the process you just did that needs improvement.
I just interviewed a guy who had no answer to one of my favorite questions/statements to ask: "Tell me about a time when you showed initiative in the workplace."
Down the road, if you are asked this question, you could bring this situation up. It's a great start to that kind of story...
"I was hired on as a temp worker at this company, and the job was supposed to take three weeks. Well, I finished it in one week. But instead of trying to stretch the next job out to fill the time, I found "X" and "X" and "X" that needed to be done, and a new way to do "X". I brough these ideas to my manager after working out a plan and...."
I'm not sure what kind of job you are doing, but maybe this helped...?