It may not take us a long time to get to Mars, but on the off-chance that it does take several years for humans to get there and back, we're gonna need a bunch of food along the way. NASA's been thinking about it, and they've got some menu ideas.
Over the course of a five year trip to Mars and back, one single human would eat about 7,000 pounds of food. Obviously, it doesn't make a lot of sense to try to pack all of that with you in freeze-dried and tube form as is currently the case on the ISS. Instead, the way to go is to grow your own food along the way, using hydroponic greenhouses integrated into your ship.
Here on Earth, growing food is a minor miracle: you can bury a little tiny seed, come back in a couple months, and preso, you've got food. In a closed system like a spacecraft, though, there's no way to get anything for free. If you plant a seed, every little bit of growth sucks down energy and material, and the entirely of the mass of the resulting plant has been stolen from elsewhere on your ship. But if you do it right, this can be a good thing, since it sets up what's called a bioregenerative system, where humans feed plants just as plants feed humans. We'd provide fertilizer and carbon dioxide and gray water, while the plants would scrub carbon dioxide, generate oxygen, and purify water. Oh, and then we'd eat them.
Getting a bioregenerative system set up takes a lot of forethought, and in space, there are lots of other issues to think about. You want your plants to be almost entirely edible, grow well with minimal tending, and be small enough that you can pack a bunch of them on board a spacecraft. Ten different crops fit the bill, including lettuce, spinach, carrots, tomatoes, green onions, radishes, bell peppers, strawberries, fresh herbs, and cabbages. Of course, that's not exactly a balanced meal, so some amount of supplies will have to be packed along, but if you can reduce bulk cargo mass by even ten or twenty percent, you'd make room for a whole bunch of extra science equipment. Or fuel. Or laser canons. Or me.
Below, check out a video that NASA produced a few years ago, where they talk about some of the other options for long-duration spaceflight munchies.