It doesn't get more handheld than this

adafruit industries offers the drawdio kit which will turn a pencil into a mini synth! Here's what their site says:

Drawdio is an electronic pencil that lets you make music while you draw! It's great project for beginners: An easy kit with instant gratification! Essentially, its a very simple musical synthesizer that uses the conductive properties of pencil graphite to create different sounds. The result is a fun toy that lets you draw musical instruments on any piece of paper.
Runs on a single AAA battery for many hours (even 'nearly dead' batteries will work)
Use any pencil - mechanical or plain. The kit comes with a 2B pencil, the softer the lead the better
Ridiculously fun for all ages