I've wanted to try a butt plug for so long! My girl is into anal, and we're going to get ourselves a nice butt plug (or two) really soon. I'd love to dominate her in a plug or have her put me in one, either way it sounds fantastic.
We regularly use a vibrator in our diaper play. It's sort of shaped like an "L" with the short leg at a 70 degree angle, with a large, rippled head at the end of the short leg for insertion. She slips it down her diaper and inserts it, turns it on, and we dry hump like this. For her it makes the entire diaper vibrate, and when we press against each other it feels wonderful. It's really a struggle not to cum in this position, but typically this is just foreplay.