"down to earth" sounds a bit broad and vague. Korean adjectives that might be translated to that tend to be a little more contextualized, like whether the person is down to earth in manners, tastes, value system, etc.
Here are some words that come to mind.
소박하다 = simple and modest in taste and lifestyle. (not luxurious)
털털하다 = easygoing and not particular in manner and taste. (not highbrow; lacks finesse)
수수하다 = plain and simple in looks and style. (not flamboyant or luxurious)
소탈하다 = open and unassuming in one's values and social interaction. (not fastidious)
So if you're referring to someone's lifestyle, 소박하다 might work best. If you mean how they get along with other people, 소탈하다 would be better. If you're describing someone's manner and taste, 털털하다 might also be a good choice. 수수하다 is most often used about appearance but can be applied to personalities too.