This DIY Ornament Kit is just the right size (6 cm diameter) for a Circuit Playground Express, 500mAh LiPoly battery, and however you'd like to dress up your ornament! Your local hobby shop fluff puff balls and other materials will diffuse those on-board LEDs nicely.
Of course you can very easily see the rainbow NeoPixels, and the onboard sound, light and temperature sensors work just fine. For capacitive touch you can use some conductive tape and attach it to the inner surface to create a capacitive touch pad. The microphone and speaker work fine too, they're a little quieter than if they weren't in a ball but you can definitely use them! Since you can't press the buttons easily, try using the accelerometer to detect when the ball is shaken to make it do stuff!
Doesn't come with either the Circuit Playground Express, 500mAh battery or Lipoly charger - so pick those up too!