I think it would depend on the person's goals. Since you said it's for anyone not competing any strength sports, then I'm assuming that the person's goal are body composition oriented.
In that case, then I think the bare minimum in the smallest space possible (i.e. a very small apartment)would be a pair of olympic (maybe even standard) adjustable dumbbells. The plate would have to be 2.5s, 5s, 10s, 25s, 35s, and maybe 45s. But I would also have to make 2 pairs of "homemade" 1.25lb weights since loaded progression on dumbbells would be tough. Perhaps the barbell would be better for mass-gaining purposes, but, given a very small space, a 7ft. BB is just too big. Maybe one of those "short" barbells and some olympics plates might work too.
If possible, I would probably also add in the doorway chinup bar since it's inexpensive and does not take up space (I use mine to hang some clothes sometimes aside from doing chinups). A sandbag and weighted vest would also be good since they don't take up space. The only problem would be doing weighted chinups, but I guess that could be solved by "practicing" 1-arm chinups. The same would apply for dips. They could probably be replaced with 1-arm pushups.
It's funny that you bring this topic up because I just started my spring semester at school and my schedule is all screwed up. So my workouts are at home whenever I get a chance. All I've been using is a barbell with weights, an exercycle (I think that's what it's called) for HIIT and warmups, a jump rope with bodyweight exercises (including the pullup bar in my doorway) for "roadwork" type GPP, my sandbag for just random lifts, and a flat bench.