I got this idea from a story I read somewhere on the internet (was a good one actually, just looked for the link but can't find it anymore
), but I think it might suit your likes, so... there you go
All you'll need is a cellphone, a bus ride - and your imagination. First: Dress up a bit. It doesn't need to be anything fancy, maybe a light summer dress, a short skirt and a blouse, or a low cut top and a nice jeans - it depends how you want to go about this dare, but it should be something you look really good in; not slutty, not too sexy, more the innocent-but-tempting "Girl next door"-look.
Now get on a bus (the longer the ride the better) and sit down close to someone - either next to them, back to back or face to face with them (face to face probably works best) - and pretend you're talking to your boyfriend/lover on the phone. All you have to do now is to act like your "boyfriend" is starting to flirt with you, making you giggle and blush a bit, and eventually even starts talking dirty to you - and of course you have to answer him and play along, reluctantly at first, but (of course) you get more and more into it over time...
That's not a very good explanation, so let's make it a bit more specific: imagine you're getting on the bus, talking to your lover over your cellphone. You sit down facing one or a group of passengers, male or female, roughly your age, maybe a little older... You're lover starts flirting with you, talking about the last/next time you met, telling you that he would like to have you there with him right now, telling you what he would like to do to you right now... Eventually he starts asking questions, for example what underwear you're wearing right now, if you're "looking forward to seeing him" as much as he is - use your imagination to create an interesting "dialogue" and make sure the passengers next to you can hear some of your "answers" (for example: "...the black lacy ones" -pause- "yes, the ones from the picture..." or "oh god, I can't say that!" - "I'm in public!" - "...yes, I loved it... I loved your taste...") and can see (from your bodylanguage) that you're rather emberassed at first (giggling, blushing, looking around nervously, ...) but get turned on more and more over time (rub your thighs together, shift around on your chair, bite your lip, ...). I'm sure you get the idea.
Try to make the people around you believe that you're having a steamy chat with your lover that's getting you more and more worked up, until you can hardly contain yourself anymore - if you want to be extra daring, you could even start touching yourself in front of them, of course acting as if you're trying to hide it from them - if you're wearing a jacket, you could open it a bit and slide your hand into it, pretending (or not, that's up to you) you're playing with your breasts, or you could put the jacket over your lap and pretend (... or not
) to reach beneath it to secretly masturbate.
Of course you don't have to let to hear them every single one of your words, but you should make sure they can catch some "interesting" parts of your conversation. You don't have to be too straightforward either (unless you want to, of course), some suggestive hints would do the job already, if you're bodylanguage is convincing enough. Just use your imagination to get into the heads of the other passengers; make them think about what underwear you might be wearing (or make them think you're not wearing any at all), make them think about the last time you had sex (or the next time you will have) or make them think about the last time you masturbated - get them to imagine you naked, having sex, masturbating, showering, whatever; simply try to get them really turned on - give them something they'll keep thinking about for a long time, something that'll make them remember "that girl on the bus": become their fantasy...
In the end, hang up, look around as if you aren't sure how much the other passengers have heard and as if you're really emberassed, and get off the bus - my guess is that you won't have to fake the blushing anymore by that time