Water. It's often free from the tap and you can keep it as long as you'd like. It's a much healthier option than soda or coffee! As far as actual foods go... root vegetables like potatoes, yams, turnip, onions, carrots and squash keep for a very long time and are very good for you (not to mention delicious!) If you're going for potatoes try to avoid generic white french-fry making potatoes. Russet potatoes, and potatoes with color (yellow, red, purple, etc) are more nutritious for you. Also some green leafy veggies like kale and cabbage can last a long time and there are many things you can grow at home with very little effort.... dandelion greens, wheat grass, blueberries, black berries, fruit trees... the world's your oyster.