When I was in the 3rd and 4th grade, there was an extra-credit program called "The SRA Reading Laboratory", which was basically a box of large index-style cards divided by various subjects, and each subject was divided by difficulty levels. If you finished with your in-class studies, you could go to the back of the room and select a card from a subject and level. Each card consisted of several paragraphs on the subject matter, followed by short quiz to quantify how well you read and understand the subject.
When you successfully completed each card within the subject and level, you could progress to the next subject and/or level to continue.
I believe that this program is what stimulated by love for reading, and, once I progressed into the 4th and 5th grade, I started spending all of my spare time in the school libraries.