A Circuit Playground! Of course, this is essential, and is required for all lessons.
Micro USB Cable any length. You probably have one of these around, they're the most common USB cable.
Make sure its a data/sync cable!
If you'd like to power your Playground for portable use, you can use a 3 x AAA battery holder with a JST PH-2 connector on it. These holders even come with an on/off switch so you can save battery when not in use.