I've got three kids who are going to be 5, 3 and 1 this summer (well, the 1 is already 1). The 5yr old in particular is getting to an age where she can appreciate the computer in more than just "moving the mouse around" ways, and I want to find time to encourage that. She's also begun asking questions of the "why is the sky blue" variety, so it's good to see that she's inquisitive and I want to encourage that, too.
Macworld has a surprisingly not-very-Mac-centric list of stuff to do with your kids over the summer. I really have to get in the better habit of letting my daughter play with the computer instead of sitting in front of the tv. The problem is that the 3yr old demands equal time, so it's hard to find things that will occupy them both without being too trivial for one and too complex for the other.