Food, but not Jell-o salads (all too commonly brought in the South). Depending on the person and your closeness to them, stopping by and talking or being there is considered good, but be on awareness as to whether that's working for the particular person.

If the 19 / 20 year old has no other family or people they're close to, it might be worth offering to sit in on any consultations with funeral directors to prevent them from pressure sales tactics ("Your mother, of course, would *want* to have our top of the line coffin...it's only N hundred/thousand more...").