I have a cake to do for my niece and I want to make a 3D Elmo in Rice Krispies. How far in advance can I do it and how to store it?

i just made one yesturday for the first time also, i let the pieces of the body dry first, then attached them together using dowels/toothpicks, then late last night i piped his fur and placed him in the fridge. He looks pretty good this am, i'm using it for my son's cake on wednesday...i've read some people make him a week in advance to harden nicely. HTH

I started mine about a week in advance and just stored it on the counter without any major issues. (Although, I knew that even though it was going to be technically edible it was unlikely that anyone was going to actually eat it so I didn't care if the treats got stale.) If you want it to stay fresh, I would imagine that you can wrap it in plastic and try to keep it as air tight as possible.