The human voice. Channel your inner child and sit in front of a good mic (in a good room) for a bit and make the weirdest sounds you can think of. Roar, moan, cough, flutter your tongue. Put the mic againt your throat...whistle...blow raspberries... Make as much material as possible.
Now repeat all of this with a friend of yours who is the opposite gender, or just has a very different voice.
If you can, record all of this at 24bit 192Khz — or at least the highest sample rate your gear can muster.
The perk: you don't have to leave the studio, buy any props, or lay out newspaper :).
Cheers,
~Matt