"He rides a bike all day"? "cykler" is "to ride the bike", isn't it? Not horses, motorcycles or broomsticks? "To bike" appears to be present in British English and is informal, at that, according to Google, and "He rides all day" the other answer, still calls for an object, outside of a context.
"He rides a/his bicycle all day" to be very clear. Or "He bicycles all day". Right?
I wrote "he rides the bike all day" and Duolingo tells me that it should be "his bike" but you know... the Danish sentence says nothing about who owns that bike.