i didnt read what the other people suggested or what you responded with, but before my computer just froze right now,
i wanted to tell you this...

i think that you should just pretty yourself up and wear something tasteful like what you would if you were personally representing one of your OWN "special" kid if you had one.
id strongly suggest against using costumes and such because that would only be making fun of the event by bringing amusement into it. and to wear a costume would suggest that you are ashamed to help the special olympics. there is nothing funny about a disability.
i think it should be RESPECTFUL and not AMUSING.

good luck, have fun.