Here are some idioms:
The best of both worlds: means you can enjoy two different opportunities at the same time.
Example: By working part-time and looking after her kids two days a week she managed to get the best of both worlds.
See eye to eye: this means agreeing with someone.
Example: They finally saw eye to eye on the business deal.
Once in a blue moon: an event that happens infrequently.
Example: I only go to the cinema once in a blue moon.
When pigs fly: something that will never happen.
Example: When pigs fly she’ll tidy up her room.
To cost an arm and a leg: something is very expensive.
Example: Fuel these days costs and arm and a leg.
A piece of cake: something very easy.
Example: The English test was a piece of cake.
Let the cat out of the bag: to accidentally reveal a secret.
Example: “I let the cat out of the bag about their wedding plans.”
You can’t judge a book by its cover: to not judge someone or something based solely on appearance.
Example: I thought this no-brand bread would be horrible, turns out you can’t judge a book by its cover.
Getting a taste of your own medicine: Being treated the same unpleasant way you have treated others.
Example: When the boss experienced a harsh critique from his team, he finally got a taste of his own medicine.