Going bananas
Going bats
Going bananas
Bats in the belfry
Don't put all your eggs in one basket.
Three sheets to the wind.
We're all in the same boat.
We're all in this together.
The lights are on but nobody's home. (Used to suggest that a person lacks intelligence or awareness.)
One in the hand is worth two in the bush.
Pope of the Dictionary.com Forum
I like to turn them into a sort of just-for-fun quiz, for example:
Don't throw the baby out with the bathwater.
A. Don't waste water.
B. I hope your child will be successful.
C. Don't force your child to do something.
D. Don't miss an opportunity just because of some parts you don't like.