What is the correct idiom to mean that something is a really good invention or activity?

What does 'let the cat out of the bag' mean?

The meaning of 'kill two birds with one stone' 

You're better off being an ignorant person

You're better off not knowing about something

Happiness is achieved by emptying your mind

When your hat falls off, you can do something

The wind is blowing and your hat falls off

Take out the champagne--you're going to celebrate

A storm is just the right size and intensity

The problem is so small it could fit in a teacup, hence the phrase: a storm in a teacup

You still have coffee, what are you worrying about?

Don't complain about something you can't fix. 

The milk was sour, so who cares if it got spilt.