Basic limbering exercises such as touching your toes and stretching your quads and neck rotations are great for beginners. You can do toe touches with a club or without. Many of the basic exercises that runners do before going for a jog are great for the casual golfer as well. Quad stretches, in which you grab the top of your foot with the palm of your hand and bring it behind you, will help keep your legs fresh for the round.
Neck circles are a good place to start for above-the-waist exercises because a kink in the neck can be a real pain to any golf swing. Grabbing your elbow with the palm of your opposite hand and pulling it toward your sternum is good for loosening up your shoulders and upper body. Limbering up the back is vital to successful club rotation and power, but be careful. Go easy on the back so you don't pull something before you've made it to the first tee.
Before you hit your first ball, take a 7-iron, which has an ideal weight for swing exercises and its length ensures you won't overswing and pull a muscle. Go through your swing in a casual manner, not stopping at the top of your backswing or your follow-through. Keep doing this without too much exertion. This will loosen up your body and get the muscles involved in your swing arm and ready for action.