Without opposition to having something inside for the sake of going harness free; I'd recommend the Feeldoe.
My only two functionality complaints about it, are that the bulb end is big if you're a small person; and that I have to put her butt up on a pillow to get a decent angle. Otherwise, the thing is fscking brilliant.
Otherwise, a strap on.I know I can get off sometimes just by being in the male role during intercourse. If you don't want her doing things in your private area, you just need to reassure her (constantly) that you really are getting pleasure out of it, and show a little bit of enthusiasm for sex. If the issue is about you being happy, then reminding her that you are should take care of that.
Also, consider talking to her. It might be a trust issue (her thinking you don't trust her), or something else that's not getting talked about.