A few peas short of a casserole
A few keys short of a piano
Not the brightest bulb on the Christmas tree
The gates are down and the lights are flashing, but the train isn’t coming
Room temperature IQ
Couldn’t pour water out of a boot with instructions on the heel
Not the brightest crayon in the box
One twist short of a slinky
Depriving some village of its idiot
A few threads short of a sweater
A few sandwiches short of a picnic
Driveway doesn’t quite reach the road