I was thinking one or two balls depending on the size... It could be a mix of crafted and purchased items, like gum ball machine prizes, temporary tattoos and stickers! When would be the best time for everyone to do this? Sign ups after Jan 1? What about turn around time, would 2 weeks be too long? I will organize, unless there is someone else interested

We could do themes... pirate, learn a new craft (with a tiny instruction book in the middle!), party/holiday/birthday in a ball (balloons, candles, etc.. wrapped in streamers!). More ball friendly crafts: knitted/crocheted/sewn hats/headbands/accessories, handmade jewelry, magnets, keychains, bookmarks, origami, small candles, a charm bracelet with the charms wrapped in layers and the bracelet in the middle, candies...
coffee_tea_me, check out the notmartha tut and google image surprise balls to see some examples of how intricate they can get (I checked the google images before I posted this to make sure you didn't get any not so pleasant surprises... Everything seems to be SFW

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Sadi27, I certainly hope you can play along, it looks like it's going to be a good time!