To make it easier to change colors when the Circuit Playground Express is in the bag, extend the Pin A5 sensor with a loop of wire. You can simply clip the alligator lead to the pad on both ends as seen here.
Plug in your battery, make sure the slide switch is set to the left, and then squeeze the wire while holding the board in order to trip the sensor and change colors.
You'll use the salt to diffuse the light and make the bag look like it's full of pixie dust. To keep the salt off of the electronics, place them inside the plastic bag, then place all of that into the pixie dust bag.
Now, fill the pixie dust bag with salt. Instant pixie dust!
You can hold the bag's wire loop and then squeeze the pixie dust to change the color! Your capacitance is measured by the on-board sensor, and it works best when you have a hand near the board and the other squeezing the wire.
Now go off and spread magic -- maybe you can even teach someone to fly!