Which highlights what I'd really like to see added to the ISS - a farm module. Test a farm module on the ISS, getting the concept ready for a Mars mission. Do we really plan to send something on the order of 2 years of consumables on a Mars mission, recycling only the water? We need much more complete recycling, and we'll need it for any permanent presence anywhere beyond Earth. For that matter the only reason we don't need it on Earth is because we've got this giant biosphere that has handled the details pretty well for us, up until the past several decades.
I rather like the idea of such a farm module even on Earth. No doubt it would be designed for compactness, efficiency, and minimal hand-holding. Sounds good to me - put one of those in the back yard and cut the grocery bills. (I realize that the initial outlay is likely prohibitive, but the idea is neat.) There are also likely places on Earth where such a thing would be worthwhile, say Antarctica or other inhospitable locations.
(Note that I didn't say that a farm module would use sunlight - that might not work for Mars, and probably not beyond.)