Hmm!
A my library (where I work as the childrens librarian) we also have a summer reading club.
Our rules are simple, read 5 books and you can chose one book to keep. You can repeat this as many times as you like and keep it up all through the summer holiday. When you have read your 5 books, you turn in a card with your name and address. This card acts like a lottery ticket at the end of the reading club when we draw winners from all the cards turned in. So the more books you read the greater your chance of winning, but you can win even if you just turned 1 card in. Since every kid gets to chose a book for every 5 read those that really like reading gets rewarded with more books and a higher chance at winning the lottery.
This works fine for us!
