"My sponsor told me not to let my mind wander...it's too little to be outside by itself."
I definitely have to remember this one.
Not the brightest crayon in the box
And on the heels of this one:

One wheel stuck in the sand
One brick short of a load
Not the sharpest pencil in the box
His/her elevator doesn't go to the top floor
The light's on but nobody's home
He/she's not playing with a full deck

Those are only a few that I can remember. I'm sure there's alot more.