The Monochord is particularly well suited for a sound meditation.  It's easy to keep a continuous bed of notes going for an extended period. 

The tone, volume, and character of the sound can evolve over time by adjusting where and how you stroke the strings.  The sound is stable and continuous, but varies enough to make it a a natural and easy focus of attention for meditation.  

Here is a video of a 12 minute monochord meditation.  Put some headphones on, get comfortable, and enjoy.