OK, this one is for "special occasions" only, the reason being is that it takes a bit of preparation time and that the sensation received is intense to the point of being overwhelming.
Obtain a metal vaginal speculum (start with size small, those metal edges can and will make you say "Ouch"!) or a metal anoscope.
Obtain a metal vibrator.
Spend a bit of time considering what will be your favorite position. Mine was on my back, legs in stirrups and well spread for easy access.
Spend some more time getting your bum well lubricated and relaxed. Pay attention to other body parts as required.
Gently insert the speculum and open it and lock it in place. When you apply the vibrator to the speculum the sensations travel up inside in an amazing fashion. It's also quite noisy with the two metals touching so you're really going to think you're undergoing quite a procedure.
If you're the curious sort, a mirror and small flashlight have something to add.
About this time other body parts may be demanding more attention too.
Naturally, as part of the foreplay for all this and to build the anticipation, an enama beforehand is a plus.
Hope you like my suggestion!
- mcclane