The shower is also a non threatening place to begin that kind of play as well, its already full of water, a lot of people pee in the shower anyway when they are alone. It is a place that will be the least threatening. Try waiting til one of you has to pee or mentions it in the shower and then jokingly toss out what if we just pee here. That may tell you a lot about how your partner will view the idea.

Talking about it like Aesop says is definatly a must. It is hard to start any conversation about something that is new and you may be afraid of a negative reaction. Try bringing it up in a joking way eg, you wanna hear somehting funny I did when I was a little drunk one night...or tell a story involving two other people and see how the feedback is.

You can also bring it up in one talk and then even if the response is not what you might expect you can let your partner think about it and bring it up later again and see what they think then. Good Luck.