Tufnell Park playing fields are, I was told, what is left of what would have been a larger green space before it was all built on. Did you know the Holloway Odeon is on the site of the Lord of the Manor's (or somesuch) residence?
I think if you walk out along the Grand Union Canal beyond Grenwich you get to proper still in use old shipyards and warehouses - might be quite interesting visually...