Reading Is Fundamental is the oldest and largest children's and family nonprofit literacy organization in the United States.
The RIF web site has a Parent Section with all kinds of tips and activity ideas to help you motivate your kids to read.
The section for kids, Reading Planet, has all kinds of activities and online animated stories.
RIF is a great resource that encourages kids to read. I suggest you add using the RIF website to your list of summer reading activities.