I'm not sure how well known this is(so it may be only news to the newbies), but press F9 twice in a single player level so you can see the pos, then type JJ as if you're about to type a code, notice that the JJ appears on your screen, now type any code you want(to activate the code) or just plain nonsense(in the screen shot provided I typed jjscreenshot) if you typed nonsense use the backspace key to backspace whatever you typed. Cool, no?
Screenshot-