Preparing to kayak is very similar to preparing for sports like wrestling or gymnastics. Kayaking provides a great workout of the upper body. When performed appropriately, your torso really gets a great workout. The thighs and feet do isometric exercises to hold you in the boat. A light warm up and stretching regimen will help you avoid potential injuries from sudden motions like bracing.
If you remember your high school gym classes, they probably had you do some light warm-up exercises to get the blood flowing and to warm the extremities. Back then, they may have had you do some push-ups for example. Another nice warm-up is light jogging or walking. Simply unloading our boats and carrying to the put-in is often good enough for a light warm-up. Don't forget to take it a bit easy after a long lunch break and warm up gradually there as well. I like some easy ascension practice to warm-up.
Only after you have warmed up your major muscle groups and especially the shoulders should you even consider stretching exercises. I would concentrate on shoulders and torso stretching for this sport. There are lots of ways to do this. In calm water, try to twist your torso and reach your rear grab loop. You will notice that many paddlers do torso and shoulder stretching in their boats with their paddles in the air, this works as well. You can also try paddling your boat on one edge for a distance and return on the other edge. If you have really good balance, it is fun to pretend you are a C Boater and practice paddling with both blades on one side. For shoulder stretching, I like gentle streching my bent arm against a tree. Once you are really warmed up and well stretched, consider a roll or two - especially on a challenging run. Each person will develop their own regimen for loosening up the shoulder and torso muscles. I would hold off on the hard core play moves until you have completed your warm up and streching regimen.
Here are some useful extra resources: