Most libraries have really neat summer reading programs.

My local libraries gives entrants a free book just for signing up, and a prize for reading five books over the summer. (For grownups the prize is a pint glass.)

As mentioned, libraries are awesome. I recommend making a habit out of regularly going to the library with your children, even if they can't read yet. At our library they have a large selection of children's books that are meant to be read to children rather than read by them. Start with these and make reading a normal part of your day. When your child asks you to read to them, try to make that possible. This might implant the habit of reading in your children.