I just did a random google search for something that I had never thought of before and wasn't sure existed, and was pleasantly surprised. While many keyholders do wear a key on a chain around their necks, many don't for various reasons. Here's an alternative that can be much more subtle... https://www.google.com/search?q=lock+and+key+stud+earings&tbm=isch They're variously obtainable as two keys, two locks, or one of each. They would make a nice present for a birthday, Christmas, or "just because".
I like that. I could see me wearing a set of the locks and Mistress wearing the keys. Might make for some interesting conversation starters.