Normally, a partner or the male himself can retract the foreskin, slowly or brusquely, depending on personal preference. But, if you don't know, better to go slowly as the glans can be very sensitive on uncut males. The same applies to the frenulum - the piece of skin on the underside of the foreskin that seems to connect it with the upper shaft of the penis. This should be one of the most sensitive parts of the male body, certainly on the penis itself. Rub it gently or vigorously depending on the males reaction.
You can pull a foreskin down as far as possible while holding on to the shaft of the penis. This can cause a pleasurable sensation for the male, but be careful not to be to energetic and damage or hurt the frenulum, which will be pulled very taut while doing this.
Other possible foreskin play - instead of retracting it, pull it upwards so that it becomes elongated and stretched.
A partner can also insert a finger into the opening of the foreskin and then sort of 'feel around' under the foreskin. This should be very sensitive. Also be careful with sharp nails. No need to cut or damage any part of the anatomy, especially the frenulum.
You can also 'pinch' the opening of the foreskin between two fingers, roll it up a ways, or just play around with it.
These are all things to do in more or less normal sex play. When it comes to medfet doings then all sorts of fantasy scenarios can be explored: a close examination of the foreskin, using forceps to pull it up or down, a small spatula like object to insert under the foreskin and twirl around, a small spoon also works well when inserted under a foreskin. For further medfet fantasy, pretend circumcision preparations or examinations can also be a turn on.
And for gay couples, should one or both be uncircumcised, one of the two can practice 'docking', in which a partner's foreskin is pulled over the other partner's glans.
There's probably more, but I'm not really very adventuresome. People with more imagination than I have probably come up with yet more possibilities.