If you're lucky, you might be able to sneak in one or two at USC (where I go). People just come and go as they please there. However, you have to be a student to actually reserve a slot. I don't know of any places where you can just walk in and use a practice room for free that are open to the public. You could always walk into a piano store, act like you're going to buy something, and then ask to practice on one of the pianos. I'm sure they'll let you.