This probably isn't the style of answer you were looking for... But:
[aspie obsession]
The computer based DAW(digital audio workstation), and their various accompanying VSTi's(virtual studio technology instruments).
The more time goes by, the more I feel that I am in fact a form of instrumentalist due to the technical depth, the learning curve, the potential expressiveness, and more. I feel that the computer is as much of a valid musical instrument as any.... And it is by far the most versatile, various, and deep. The learning process never ends. Mastery isn't actually mastery - you can always learn and improve more, and more, and more, and it never stops.
My recent song, posted in the stickied showcase thread, has a section near the end where I go nuts with my synth's automation, and take very, very meticulous and precise care, so as to give it a 'vocal' or 'talking' quality. Not only is that accomplished by the initial patch I put together, but by very specific, not-at-all-random movements of the various filter and distortion parameters. I included a screenshot of it for fun & profit, though it's probably unintelligible if you're not into synths and DAWs...
It took a lot of Asperger's-Style hyper-focus and time to get that good with my synthesis and sound design. However, due to my ability to grind it through with such focus, I was able to go from 'synth noob' to 'extremely competent' within about 2 years. That's pretty damn good.
So.... Why am I listing it as an "easy instrument?"
Because it requires zero manual dexterity and muscle memory!
You play it purely with your mind.
Seriously, what's the difference between a crazy synth solo and an Eddie Van Halen guitar solo? Manual dexterity. A physical trick. THATS ALL.
Respect for guys who can do those physical tricks, but personally I'm more interested in the creative elements.
>Very Mild But Fun Example<
But there's way better examples out there... Just can't think of a cherry one at the moment. It's late.
[/aspie obsession]