Always very happy to swap ideas LadyAmanda ...

The slit pocket idea is good but why not make HER do the fingering, and also insist she goes all the way to orgasm. There are innumerable variations of this as well ... such as where it occurs (public transport, standing in a queue, standing at a bar, etc), or to make it more interesting why not insist that she enter a conversation with a stranger while her fingers are working their magic, or setting a time limit (usually a long one rather than a short one).

Another thought is to make her bring herself to orgasm in a public toilet cubicle while totally naked ... and totally means totally ... that's no clothing, jewelry or even make-up.