Wild asparagus grows over a huge portion of the United States, thriving in pretty much any place with sunlight and water. It is prepared exactly like store-bought asparagus; steamed, boiled, roasted, or any other method you like.
Prickly Pear Cactus
Many species of cacti are edible in some way or another. One of the best, and better-known varieties in the western and southwestern deserts I call home, is the prickly pear. While the stem of the cactus is quite edible when boiled, the real jewel is the red or purple fruit.
If the area you're foraging in includes wetlands, cattails are a very real potential food source. Nearly all of the plant is edible, with boiling required on many parts to make it edible. Dig up the roots and boil them. Boil the leaves and stalks. The head of the plant can even be cooked and eaten like corn on the cob.
These three common, widespread edibles are high in nutritional content, simple to prepare, and can taste quite good. While restricted in range, prickly pear fruit are a wonderful source of nutrition, water, and wonderful flavor. The other two plant species are so widespread as to border on being universal, and can help save your life no matter where you're trying to survive.