We finally bought a tablet last week (an Android, not an i-thingy). In part so that we can learn how to program apps for the tablet, but it's getting a bit of a workout with other things too.
One app that's proven a hit is the Brain Pop app. Every day it has a new featured movie - a short educational animation about whatever is the topic of the day. You can then choose to complete a short quiz based on what you've watched. The quizzes are multiple choice - some are comprehension based, language based, reasoning etc.
So far, the topics Billy has watched include Mars, bacteria, bullying and constellations. All in bite-sized snippets of learning.