walk around with (one's) head in the clouds

Translations
ходить с головой в облаках
Someone often walks around with their head in the clouds, dreaming of adventures.
Кто-то часто ходит с головой в облаках, мечтая о приключениях.
Definitions
To be distracted, dreamy, or not paying attention to what is happening around you, often because you are lost in your own thoughts or fantasies.