I know this is an old post, but it is still a good source for ideas.
Humiliation sub here. For humiliation, I think it's got to be something others know about. For example, to go out in public with a butt plug in place might be embarrassing to the person, but not humiliating. Maybe a bit humiliating because your domme knows and you are out in public not a private place. If you tell someone you have a butt plug in, that would be humiliating to most people.
For me a few things come to mind. Once I was in a sex shop with a woman I had met online and we were just getting to know each other. She knew I was into a bit of humiliation. The shop only had a couple of customers and a female attendant. We were maybe 20 feet from each other when she holds up a dildo and nearly shouts to me 'do you think this is too big for you!' I nearly died. She then asks the female attendant about larger sizes, strap on, vibrating functions as I am standing arm in arm with her so there is no question about who it is for. That was very humiliating. One other customer was even listening in.
Another time in an different shop she asked the sales person, a male this time, for personal lube for anal saying her strap on had problems going in. Nothing more humiliating than being talked about that way by a woman to a complete stranger. We started having fun this way for a while, just things to humiliate me in public.
We visited a sex shop with my balls tied up with a cord hanging out the back of my pants. It trailed a bit, maybe 12 inches or so, out the back. It didn't look like much and not real noticeable. She had me wander over to look at some magazines when there were others there. When she felt others could see, she came up behind me and pulled on the cord. Of course it gave a good tug on my balls. She then said 'I told you not to look at these magazines!' Mystery solved as to what the cord was for. Of course there were some snickers and smiles as they quickly figured out what was at the other end of the cord. You could try this with the cord attached to a butt or vaginal plug. You might have to loop it around the front to prevent pulling it out, but you get the general idea. Maybe have her wear one of those horse tail butt plugs with the tail hanging out below the level of her dress. I shouldn't take too much imagination for others to recognize what it is. I'm sure your sub will be blushing the entire time.
I always thought the idea of a woman in public wearing pasties with tassels would be humiliating to her, but I'm not sure that is legal in most public places. Maybe at a nude beach. Same as being shaved at a nude beach. It all amounts to what is beyond the subs comfort zone.