I noticed that some sentences in this topic only except "sit" and others only accept "sit down". I think both should be acceptable, especially if "ned" isn't used in the danish sentence. Regardless, consistency would be nice.
Does "We sit on the beach" imply a movement? "Sit down" seems to be the better option here.
The translation offered is not correct. The Danish sentence states that we sit at the beach whereas the English version indicates that we "sætter ned på stranden". Small difference but the lack of accurate translations at this level of the course takes out quite a bit of fun.