Good luck with your idea. Quizmonkey is right, though. It's amazing how many things of potential interest we walk past every day without noticing. As a bit of a time saver, you could try a library for a start to see whether there are any books (usually paperbacks) with suggested walks in various parts of London, or you could buy one. These usually detail the route, and mention items of interest. Depending on how long you want people to spend on the hunt, you can pick out one, or perhaps two adjacent ones. One essential is to do the route yourself, partly to find out how long it's likely to take, and partly to make sure that the things you're wanting people to look for are really there. Then all you need is a fine day. Afraid I can't help with that one.