In previous years, I've encouraged the kids I nanny for to do the Barnes & Noble reading challenge, as well as the one with the local library. From B&N, they earn a free book, and from the library they earn a series of three prizes, usually a reusable book bag, a book, and a choice of either a book or a ticket to Elitch Gardens (our local amusement park).
I love the reading challenges. They encourage kids to read more, which is really what you want them doing with their summers anyway, and the kids love the free swag.
For more information about the Barnes & Noble reading program:
Barnes and Noble Summer Reading Program for Kids
Also be sure to check your local libraries. A lot of times the kids don't actually need library cards, so you might be able to do multiple programs, although I always insist that the kids can't count the same reading hours or books for two or more programs at once. Every prize has to be earned with new or additional reading. That's just my rule, but it seems only fair (and it encourages more reading).
I can't believe it's already almost summer. I used to love reading all summer! Who else spent their summers in this way?