My sketchpad: a breadboard.
And these are my drawing instruments: jumper wires.
The great thing about them: they are easily removeable. So you can quickly sketch another idea if your first one doesn't work out.
Below, a sketch in action. Don't like it? Pull the jumper wires out and start over.
On the Internet:
- Breadboards for Absolute Beginners (from Instructables)
- How to Use a Breadboard (from Sparkfun)