About 700 plants in North America are poisonous and many will cramp your style by killing you, including that rhododendron tree in your backyard. Pelton's suggestion is eating bugs, or what is scientifically called entomophage. In many parts of the world, eating critters is considered normal. In fact, there are 1,462 known species of edible insects. Probably better than some energy bars on the market.
Bugs are low in fat and high in protein.
Remove any nasty jaggy things like grasshopper legs or beetle horns before munching. Soft bugs like moths and caterpillars can be roasted. Scorpions are yummy if you chop off the stinger and roast them. Cockroaches need a good enema and smell bad, wasps and bees need to be boiled thoroughly to break down the poisons and soften the stinger.