As mentioned, Yonge and Wellesley has a number of excellent spots. You'll need to provide details to get specifics from there. A Bay Street option is the New Yorker Deli (at Charles on the west side), which has excellent sandwhiches and other quality deli food. I once had a good sandwhich from the place in the basement of the Manulife Centre - Scattia I think it's called.
I would recommend walking one street over to Yonge and Wellesley and deciding what type of food you feel like. There are a ton of great little restaurants in this area that are usually fast and cheap. Some examples:
Red & White has great shawarmas,
Yummy's BBQ is good cheap korena food,
next door to that is the Bowl or noodle dishes.
If you want carribean there are a few different ones in the area that all seem to serve the same food and taste good.
Pi-Toms is good thai food,
Koyoto sushi is good for sushi of course.
If you have a craving for something it might be easier to tell you where to go given the type of food that you feel like.
I strongly recommend that you do not go to the cafeteria in the government building. It is a big ripe off for REALLY bad food. You will pay more there then any of the other suggestions I have given you.