UA is walk-in, and open Wed to Sunday, and they include rentals.
Hill437 is a private field, and not really newbie-friendly (in the sense that you're expected to be ready to go, and know what you're doing).
TTAC3, at Ossington and Dupont, is pretty newbie friendly, and runs games on Fridays whenever the building is not in use for something else. Usually 2-3 times a month.
Like pusangani said, keep an eye in the Ontairo games section. UA is the only drop in field in the GTA. All the other games start and end at specific times, on specific days, at specific venues. Most games are pretty newbie friendly, especially if you let people know you're new.