Check about laws concerning doing any kind of business in city parks. I'll bet you need a license in most places.

You're right that it's a bit more difficult to have someone sit for you, or stand for you the time it takes to make a sketch, but I agree it can be done and you'll make a fair amount if you're any good.

Check out to see if there are any pavement artists working around the city. In cities across Europe, I've seen both of these types working in the same areas and they seem to compliment each other.

I'm originally from the Chicago area and know there are lots of places you can check out...but as I said, if you're going to be somewhere like Lincoln Park, or around Buckingham fountain (both great sites) you might have more trouble than if you hit the beaches that are less patroled.

Good luck!