Once in a blue moon
Rarely
In leaps and bounds
Fast
A whole new ball game
Another deal / Different topic
Cool as cucumber
Calm
a fish out of water
Uncomfortable
To let the cat out of the bag
To spill a secret
To be on cloud nine
To be very happy
a heart of gold
Kind heart
Piece of cake
Too easy
To bite the dust
To fail
To be in a fix
To have troubles
Commit something to memory
Remember
To go bananas
To get mad
A sore point
hurtful, disappointing
To eat like a horse
Be very hungry