日帰りって英語でなんて言うの？

"day trip" can be used to describe the duration of a trip "it was a day trip to spain by car"
or
"I took my family for a day trip to the park"

We simply refer to this as a 'day trip'.

E.g. "I went on a day trip up North of the island to visit some waterfalls."

"A day trip."

An excursion.
Leaving your home or usual place of rest to enjoy the sights and activities away from home.
A trip that spans for a few hours and not more than twenty-four hours.

A "day trip" refers to a type of a mini holiday that lasts only a day.
You would spend the day typically doing some activity, that you would normally do during the vacation time. eg: Sightseeing, travelling, doing a fun activity.

"Let's take a day trip this weekend"

"What did you do yesterday?"
"Oh, I took a little day trip to the city"

A "day trip" is when you take a small journey to a place, that is not far away, and easy to get to and back within a day or by the night time of the same day.