Investigating Arduino project ideas suitable for grade school level.
Scratch for Arduino: http://s4a.cat/
This seems a rather simple extension to scratch which runs a continuously-updated polling loop of the Arduino through the USB/serial interface. Works ok, but only supports a short list of simple devices. It would be interesting to see how difficult it is to extend to e.g. Neopixel programmable RGB LEDs.
It installs and runs simply on a Mac running Darwin 14.1.0. Does not run without major fuss on Ubuntu 14.04 64-bit. (older Ubuntu ok, 32 bit ok, so they say).
Hardware
Analog RGB LEDs seem to run $1 each or more.
Here are some 5mm ones for $0.79 each: https://www.superbrightleds.com/cat/through-hole/filter/Color,0,3,103: