Breakfast. Depending where you stay, you can have breakfast in the hotel (at additional cost) or not. If there is no breakfast at the hotel, on the way to the Banach Center will be many cafeterias/fast food. If you stay on the premises of the Banach Center, then, if you like cooking, you can make not only breakfast, but also other meals, purchasing food in nearby grocery shops.
Lunch. Usually participants of conferences at the Banach Center, go (in smaller or bigger groups) to nearby cafeterias/restaurants. Some suggestions (with locations) will be provided later on. Within walking distance, there are dozens of places to eat, ranging from fast food/cafeterias (~$5) to expensive restaurants ($100+). In very cheap places, you have to pay in cash (no credit cards), but starting from medium priced restaurants, you can pay by credit cards.
Dinner. Usually participants go (in smaller or bigger groups)
to better restaurants, especially in the Old Town (4km to the north
from the Banach Center).
Average diner starts at $20 - 30/person.