Did you know Subway has a veggie patty sandwich? They don't advertise it and it's not on the menu board, but most of the restaurants have it. And it's really pretty good - not a weird texture, decent taste, and way more filling than a regular veggie sub. You just have to ask for it!
Random secret, huh? I've heard Dairy Queen is similar - that they have blizzard flavors that they don't advertise and aren't on the menu, but you can order. So strange!
Check out this link for more supposed restaurant secrets.