thigh or calf? For calf, I stand on the pedals and drop my heels to give them a good stretch. For thigh, I ease up. I haven't tried it but I've seen some of the pros on TV take their foot out of the pedal, reach down and grab their foot and pull it against their butt (oh wait...I mean "ass"...'cause you can say ass) to stretch their thigh.

Hamstring stretch is similar to calf stretch except of bending more at the waist.