"Grab" a light out of the air with your magic thumb tip. Hold it above your bag and "drop" the light inside while giving the bag a shake. Each time you do this another light will appear inside the bag, until all your lights have been lit.
Once your bag is "full" of lights, up-end it and watch all the lights disappear. Your trick is now reset! Turn the bag right-side-up again and do it all over again.
Make this magic trick your own by adding other functions. Magic light thumb tips come in lots of colors, so maybe try using the buttons on the Circuit Playground Express to add multiple color effects.
You could also add sound effects -- play a sound each time a light is "captured" -- or have the lights dim or disappear when you shine a flashlight at your bag. You could trigger the lights to appear when the Circuit Playground hears a loud sound -- maybe when you shout "Abracadabra!". Or secretly hide some red jewels or giveaways inside the bag, so when you spill the "lights" out, they appear to have changed into jewels.
Make someone smile!