I love it when they nibble on my foreskin, stick their tongue underneath and lick around, suck on it with their lips, push the foreskin back with their lips and lightly brush it with their teeth as they push it in their mouth. It can be used as a shot glass also. Just pull up on it until it makes a cup and pour some liqueur in it and pop it in their mouth. I also like to pull it over the glans and stick it in their mouth and have them LIGHTLY clamp their teeth down and then retract the foreskin whilst keeping my dick in their mouth.
There are a multitude of things you can do with a foreskin and most of them cause EXTREME pleasure. Just be gentle. The cock head is very sensitive and with the wrong move or pressure it can cause EXTREME pain.