Quote: withouth any instruments
Yes, but I hope you're a great singer. You should have a look at 'Manhatan Transfert' or 'Bobby McFerrin' for example.
Quote: without you playing any instrument
If you're musician, virtuals instruments, like suggest kon-tiki, some of them sounds really fantastic. You can use some of them in stand-alone mode (without other software) or integrated in another music software (like cubase, logic ...)
If not ... good luck ! You can use preformatted samples, but limits must come quickly.