An Aloe vera plant, not only are they pretty and fairly easy to care for (potted in a southern or western window, watered occasionally), their leaf goo is an excellent treatment for burns and the basis for a lot of over the counter topical salves. It’ll always be there when you need it and regrow what you use over time. Just snap off some of a lower leaf and squeeze out the goo on to a burn. Just… don’t eat it, not good for eating.

Ginger and garlic steeped as a tea are also a fairly good symptom alleviator for coughs, upset stomachs and head colds, adding a fair amount of honey is good too. Easy enough to keep around, not super extreme like some cold medicines can be but still surprisingly effective. Ginger is actually fairly easy to propagate and grow, but, eh, cheap and common enough at any story that keeping a plant of it around may not be worth the effort.