wilco,
As above, you must look at the individual needs of the person. I like to observe, or at least speak to the teachers/parents about what the person has experienced Musicially, as well as what must be avoided. In the beginning with the younger children, I have used Orff instruments, my Mtn. Dulcimer, & my Guitar tuned to a key, and then letting them strum & pluck as they can so that there are no wrong notes & the Music is pleasing. I generally try to give different Music experiences over time, such as lower, higher, various instrument groups, wind, brass, whatever. The kids usually like to experience my small percussion/rythm instruments, many of which I made or found. Look in the kids section of the Mudcat for some examples. One which was terrific with many different spec ed kids was a long, dried seed pod I picked up from under a tree. It was stiff, but had rounded bumps where the seeds were, could be easily held, and could be played by shaking softly or strongly, or even bopping it against a hand or ear.
I think the most important things are to have a successful Music experience, and to experience different things--sounds, meters, styles, softer, a little louder, higher & lower, etc., & moving/dancing/playing/listening to Music, and most of all--to have FUN!