Hibiscus is a large family. Rose of Sharon is not exactly edible. They have certain types of hibiscus that produce a swollen calyces, and THAT is what you see as hibiscus tea and bits of them available at a lot of stores (especially the health food stores). It has TONS of Vitamin C, has mouth imploding sourness (Citric, Malic, Tartaric, Ascorbic Acids). The actual one that produces the calyces that are edible is the hibiscus sabdariffa..
The elderberry are all over the place here. They grow over 30 feet sometimes for the very old ones, 20 feet for most of the tall ones around here. The are in the sambucus family, and have a TOXIC relative here, the RED elderberry.
And loveofrose has it right: There are literally thousands of "weeds" that are edible and/or medicinal. We regularly forage for several species of mushrooms, some greens, and a few veggies when they are in season. BUT you need to be ABSOLUTELY certain of what you are foraging and eating. Poison hemlock is around here, and closely resembles a couple items we go for. I'm not dead from eating anything, or even sickened by anything we have foraged for. Because we know EXACTLY what we want, what it looks like, where it grows, etc, and learned that from others with experience. Even dandelions are edible, the flowers give you dandelion wine, the leaves are salad greens and the root is good for making teas. There are a lots of things we call weeds that are actually edible and even healthy for us.