Pinata minigame?

So my campaign has been getting a little dark lately, and I was hoping to liven it up with a short mini-encounter. I'm thinking about trapping all of the PCs into a small room full of magical darkness, and putting an animated pinata-like monster with a pretty high AC in there with them. The player to break open the "pinata" would get a reward.

Any suggestions on how to structure this encounter for maximum zaniness and fun while keeping with the DnD setting and atmosphere?