On the not very clever side:
"Not the sharpest tool in the box"
"As sharp as a sausage" (also....as a marble)
"As bright as a five Watt bulb"

Others?
"As popular as a turd in a fruit salad"
"As brief as a butterflys fart"

"You can't wrap fire in paper"
"One hand washes the other"

We also used to split two sayings, and cross-join them. For Example
"People who live in glass houses...live to fight another day"
While
"He who fights & runs away...should't throw stones"

Of course "It's a bad day for the Gaels, when the ducks are in the nettles"