Kushari is one of the few traditional Egyptian meals that is almost always vegan (other than some mezze), and many people consider Koshary El Tahrir the best kushari joint in Cairo. The location is two short blocks east of the now-famous Midan Tahrir (aka Tahrir Square, even though it's a circle...)
Service and atmosphere are bare-bones but convivial and authentic and the food will fill you up for hours of exploring Cairo. Three sizes are offered, but only the most perseverant diners would be likely to make it through the large, so i might suggest starting with a small or medium.
Cons: Only one dish available