Nairobi Yaya Centre Restaurants has lots of options from casual to upscale and in fact it has at least 7 restaurants.
Alexandre on the main floor makes excellent sandwiches and pastries in a casual setting.
Java House is a coffee house on main floor that has excellent American style breakfasts and bakes goods.
The Food Loft on the 2nd floor has 4 casual restaurants, all of them great for lunch or a quick dinner. Saffron is Indian - try the chili paneer, Primi Piatti is Italian and there is also a Chinese as well as 1 other restaurant.
Sierra is an upscale lounge/restaurant with a nice patio. They have an Intercontinental menu that has not disappointed after 6 visits. Their home-made micro brew is almost as good as Tusker.