Phun: physics simulator -- build and wreck stuff to your heart's content with this 2D physics sandbox. There's lots of videos on YouTube and other Phun-user sites that show just how complex you can get with this; but even very young kids can get the hang of it easily. My favorite part: create a shape, then liquefy it and watch gravity do its thing. The clouds that roll by occasionally are a nice touch.
Audacity: simple sound editor that lets you do multi-track recording… record a song, then add some sound effects and a beatbox track; or see what's it like singing "Row, Row, Row Your Boat" with a few clones of yourself. Some hints:
- change a few settings to make things easier: have existing tracks played while recording new tracks (so you can hear to match things up); and set recording to mono mode so you can fit more tracks on the screen and not have to look at empty ones (mic recording is mono anyway)
- start with a "click track": make a track of "one… two… three… four… one… two… three… four" to help keep the beat, then just delete that track as the final step; and start your other tracks after the first set of counts (like bands do when the drummer counts it down)
- to record multiple tracks, just record something and hit Stop, then hit Record again: a new track will be created and recording will start.
Enjoy.