What great fun! For you both!!
A wonderful way to keep the spark going. Wish I had done this when first got married and encouraged my wife to do similar things for some sexy fun. She is a confirmed closet exhibitionist, loves to be seen when she is in the right mood, but will deny it. She has to be "caught" with plausible denyability - something many of us have commented on about our wives in other threads.
We did stumble upon the exact same changing room set up at a clothing store when we were on vacation a number of years ago, and did get her to try on a few things while leaving the cloth "walls" partially open. Emma executed your dare much better than my wife did.
To your request, the changing room was one of the first things that came to mind. A slight variation would be for Emma to come out and show you whatever she has tried on, letting any others see her. The more revealing the item(s) are the better: sheer / thin tops, wrap tops / dresses that "have" to be adjusted while looking in the mirror giving a chance for other to see what is contained by the clothing. You could have a conversation about whether she can wear any lingerie under whatever she is trying on - or not. And she shows you the various items both ways... Showing you just lingerie she is trying on might be a little too obvious for it to appear accidental, but maybe...
Shoe shopping is another variation which can reveal all and sundry... Emma could be so focused on how the shoes / boots look, asking your opinion, that she forgets what the shoe salesman can see.
With summer coming, wearing thin tops / dresses and getting caught in the rain when out and about and Emma can do nothing about how transparent her clothing becomes. Might be fun to expand when / where Emma has to complete her dares. Her clothing style for work could evolve to be more revealing though still professional. Lower cut tops, slightly more sheer tops, tops that hang forward when she leans over, shorter skirts / dresses or ones with high slits in front that show most of her thigh when she is sitting, and maybe more. Wrap dresses and tops can be so versatile...
Sexy, sheer lingerie is always appreciated by anyone who is lucky to unexpectedly see it and what it contains. Emma can easily make it appear to be "accidental" and yet repeat it as often as desired. "Failing" buttons on clothing is another method; either the threads let go [as somebody had cut most of them] or the button hole somehow became too large and the button repeatedly slips through, especially when any pressure is applied. A strategically "missing" button on a top might not be noticed by the wearer for many hours... though others will notice.
Going out in the garden in just a thong was pretty daring, but fun for all. Bet your neighbors had a fun night as well after seeing Emma, the other guys did too! Nude sun bathing in the "privacy" of your back garden? Depending on how your place is set up, removing window curtains / blinds as you are repainting / redecorating would be a good way for Emma to be seen by the neighbors especially when the lights are on at night.... taking a bath, getting ready for bed or dressing for work Could go on for quite a while. We moved to our new place almost 2 years ago and still have no curtains... and for some reason neither of us are in any hurry to get any.
Please post more stories - Emma sounds like a wonderfully fun, playful and very sexy girl! Would really like to see photos of your vixen but understand her reluctance. Maybe carefully cropped photos eliminating anything identifiable?