Beyond your wildest dreams
Something that is better than you could have imagined.

A figment of your imagination
Something that someone imagines but that is not real.

Castles in the air
Plans or dreams that are unrealistic or unlikely to happen.

Daydreaming
Lost in one's thoughts, imagining things while awake.

Let your imagination run wild
Allow yourself to think freely and creatively.

Stretch the imagination
Think beyond the usual or expected.

In your mind's eye
In your imagination or memory.

Thinking outside the box
Thinking creatively or in an unconventional way.

Flights of fancy
Impractical ideas or thoughts, purely imaginative.

Out of this world
Something so amazing it seems unreal.

Build castles in the sky
Make plans or have dreams that are not practical.

Head in the clouds
To be lost in your own thoughts and not aware of what is around you.

Pie in the sky
An unrealistic promise or hope.

Paint a picture in your mind
To imagine or visualize something.

Fired up the imagination
Inspire someone to be creative.

Go out on a limb
Take a risk by using your imagination or unconventional thinking.

Spin a yarn
To tell a long, imaginative story, often exaggerated.

Stretch the truth
To exaggerate or embellish the truth.

Seeing is believing
It's hard to imagine something without actually seeing it.

Wishful thinking
Hoping for something that is unlikely to happen.