Since the iPhone seems to be every kids new favorite toy, how about doing some reading on that thing? PopOut! The Tale of Peter Rabbit is available for both the iPad and iPhone. It has interactive features like a real pop-up book and "read to me" or "read it myself" modes.
For older children, and only available for the iPad, is JFK: 50 Days which features rare video from the NBC archives. Check out Best Kids Apps for more ideas broken down by age group and category (there's an educational list, ahem).