Look into summer reading challenges at your local library. Many public libraries run summer reading challenges, encouraging readers of different ages to read a certain number of books over the summer.
You may also consider setting up a reading challenge yourself. You can offer certain rewards for specific reading milestones, such as a pizza night, evening at the movies, park outing, or another prize that would entice your child. For younger children, consider setting a goal for books read. Meanwhile, you may set milestones for chapters or pages read for older readers.