Knee flexion or leg curl is the recommended exercise, either with a resistance band or on a leg curl machine. Flexibility is very important to a surfer. The body contortions the average surfer makes to pull off radical maneuvers or even just fit the wave are beyond normal. Yoga is a very popular form of stretching and strengthening and also has a powerful mind to body relationship. One of the most important things to remember when stretching is to warm up prior to a stretch.
Muscle is similar to silly putty in the sense that it is less pliable when cold.
A cold stretch can sometimes do more harm than good. However way you train for surfing the one thing to remember is have fun . Because if its not fun, chances are you're not going to do it. Surfers are inherently fun junkies, so keep the stoke and have a smile.