This week for in my Core for Beginners Workshop I focused on exercises using stability balls as well as medicine balls or Bosu Soft Fitness Balls. For beginners I'm also a big fan of the Bosu Ballast Balls as they are a bit more stable due to the weight of the sand. There are lots of sites with great stability ball exercises but I think this one is particularly awesome! You have to register to have access but it is worth it.
These exercises may be mixed with the exercises from last week. I recommend 2 - 3 sets of 10-15 reps for each exercise.
Bridge Press with feet on ball
Bridge Hamstring Curl
Quadruped with abs on ball
Side Leg Lifts on ball
Windshield Wipers with calves on ball
Standing Russian Twist with either medicine ball or stability ball
Standing Side Bends with or without light weights
Also, if you are looking for a good exercise app, try Sworkit. Easy to use and great variety.