Scores of small and friendly restaurants line the two main drags (okay, the only two real streets) in Aguas Calientes, Avenida Imperio de los Incas and Avenida Pachacútec. There's always a proliferation of cheap pizzerias/pit stops hugging the railroad tracks. If you're looking for an easy meal of wood-fired pizza, almost any spot in town can accommodate you; El Fogón de las Mestizas (Avenida Pachacútec) and Incawasi, Inti Killa, Pizzería Su Chosa, and Pachamama (all on Avenida Imperio de los Incas) are all dependable. Menu hawkers, often the children of the cook or owner, will try to lure you in with very cheap menu deals.
For lunch during visits to the ruins, you have two choices: the expensive buffet lunch at Machu Picchu Sanctuary Lodge or a sack lunch. I recommend the latter, especially because lunchtime is when lots of tourists vacate the ruins (pick one up at Gringo Bill's or assemble one from the breakfast buffet of your hotel).
Note: This information was accurate when it was published, but can change without notice. Please be sure to confirm all rates and details directly with the companies in question before planning your trip.