For an inexpensive meal you could eat in Macy's on State Street. There is there version of a food court on the 7th floor with your choice of snadwiches, salads, burgers, asian or mexican food. The Mexican place is a Rick Bayless outlet and the tortilla soup is amazing. Also on the 7th floor is the Walnut Room where you can also have a sit down lunch. At either dining option on the 7th floor at Macy's try to secure or ask for seating near the window and watch the street scenes below.
As for Michigan Avenue, I love Bandera so if you can spend more like $15 and up per person for lunch then go for it. Otherwise Macy's is more in the $10 and below range at the food court.