Yeah you gotta feel the situation though. If you can't 100% get right behind the "making it a game" idea it will be really creepy and wierd, she will totally pick up in your tone of voice that you're just saying it with one outcome in mind, instead of just doing it for fun. Personally I just find it easier to dive in to things head first and let people deal with the consequences themselves. Whoever's reality is stronger, wins.
In David D's "Double your dating" seminar there is a guy who says he just takes them on a walk by the river, says "let's go for a swim" and when she agrees and they go down to the water, he rips off all his clothes and dives in. 99% of the time, the girl will join him, in at least her underwear, if not completely nude herself. Then his thing to do is quote them some of his poetry that he writes - but yeah, whatever works for you
You don't really need complicated tricks to "trick" girls into doing things, these "tricks" are really just tools to support your already existing confidence and mindset that whatever you do, is going to work. And that whatever happens, you are gonna have fun anyway. Like if she really isn't getting nude - you're still nude and she's cool with it, that's still pretty awesome, I've had that plenty of times myself -there's a girl perving on your nude body, that's pretty awesome