The correct preposition is "
at" — "They enjoy the day
at the beach."
If you want to choose between "
on," "
in," or "
by" — let's break it down:
at the beach → The most natural and common choice.
on the beach → Feels too literal, like being physically on the sand (but this is sometimes correct too, e.g., "They had a picnic on the beach.").
in the beach → Not idiomatic English.
by the beach → Means next to the beach, but not necessarily at it.
So in this context, the most natural choice is:
But if you really have to pick
by, on, or in, then "
on" is the closest:
If you want the most natural phrasing, though, I’d recommend: