If it's something that you want to do more than once, then it might be best just to be honest. Especially if she's usually cool about stuff like that. If you try and introduce it as a game or as something that just happened by chance, she may not realise that you actually enjoy it and would like to do it again. I would tell her upfront that it's something that you'd be interested in trying out - if she isn't a judgemental person, then what's the worst that can happen? In my experience, being upfront and open about your desires gets you what you want far faster and it's much less frustrating than trying to drop subtle hints over a long time.
If that seems a bit daunting, then introducing it as a game may be better for you. But make sure that you discuss it with her afterwards - find out whether she enjoyed it and make sure that she knows YOU enjoyed it and would be interested in trying it out again.
Hope it goes well for you
