On pain or discomfort
I think some may leap to suggesting a tack bra. This is where you would tape thumbtacks to the inside of the bra pointing inward. The bra would then be put on loosely. You would have to take great care not to place pressure to the bra. If you go this route be sure to use plenty of tacks. You will want to even overlap the tack heads. This way you have the bed of nails effect. If you do press on the bra or wear it tightly the pressure is distributed across many points so each presses little. You might want to dull the points, at least on those near the edges.
I am unsure if this will work with small breasts. I am guessing that your breasts don't rub across the inside of the bra very much, or as much as those with large breasts. You could place sandpaper inside the bra. I think you would want to use a grit between 4 and 30. You may have to experiment for the right grit. You will somehow have to find a way to fasten the paper to the bra. I think sewing is out of the question as it will likely simoly cut the paper. I suggest cutting the paper into wedges and using double face tape. It won't survive a wash. Some sources may have sanding cloth. This may be sewable and servive the laundary.
You might try course steel wool. This could be layer out on the inside of the bra and sewn in with a basting stitch. This will likely rust after each use. I doubt you can find stainless steel wool.
I doubt if you can find itching powder locally. But there are online sources and instructions on how to make it. You could just place that inside the bra and go for a walk at the mall or a bar/club.
On display
You could simply cut out the nipple area. But that is too simple and overdone. A sewing store will likely have chrome studs (hemispheres) that can be fastened into the fabric. A large one could be placed at each nipple. This would be good on a black bra if you intend to show the bra. If you plan on showing it under another garment then you could also use smaller studs across the whole cup, perhaps in a Starburst pattern. The bumps showing through a tight outer garment. You may also have to purchase special pliers to affix the studs. There are also more pointy higher studs available.
You could decorate the bra in some way and wear it as a bikini top. A tie die would do nicely. There are paints that come in toothpaste like tubes that stand out for a 3D effect. You could use contrasting or bright colors. Start with a dots at your nipples. Add circles out beyond your areolas. Then continue with circles of different colors or draw rays out from there like sun beams.
You could do something like body writing on the bra with a permanent marker. There are markers available that are designed especially for drawing on fabric. You could let your mind wonder on what humiliating phrases would fit. Place your name on the back band or along the straps. Buy a burner phone and draw "Text me at xxx-xxxx" using it's phone number. You may have some very interesting conversations.
Get feathers, fur or even brightly colored fake fur. And attach to the bra. Wear it as a top. Again circles around or rays from the nipples are especially attractive.
Is that enough?