In my post "Beginner Bodyweight Workout to Help You Get Into Shape," I gave you guys a very simple beginner routine that you can do anywhere and at anytime with absolutely no equipment.
So now it's time to release a follow up workout: Exercises: Crunch -

• Lay down on the floor with your knees bent and feet flat on the floor.
  Lift your head and shoulders off the ground, curling the body slightly forward, and place your hands underneath your head.
  
• From this position, squeeze your abdominals so that your back comes slightly off the floor.
  
• Release and return to starting position.
  Remember that the return position is the position with your head and shoulders off the floor.
  It's very important to keep your head and shoulders off the floor during the entire set.
  

Decline Pushups -

• Get a sturdy chair and place it against a wall with the back of the chair (the part that you rest your back on) facing your.
  Place your hands on the back of the chair.
  Keep your back straight as you straighten your legs out behind you.
  
• This is very similar to a wall pushup, with the only difference being that your hands on a chair.
  Perform a pushup.
  The second, more difficult variation of this movement is to perform pushups on the seat of a chair.
  
• The main idea of a decline pushup is to find an elevated surface to place your hands on while performing pushups.
  Steadily lower the height of this surface.
  The lower the surface, the more difficult the exercise will become.
  
• Keep lowering until you are able to do a full pushup on the ground.
  

Workout: 5 rounds of:

• Crunches, 10 reps
• Decline Pushups, 5 reps

Move through the workout as fast as possible.
Time the workout, and try to perform it faster the next time.
Eventually you will need to make the workout harder.
This can be accomplished in three basic ways:

• Replace the exercises with more difficult variations
• Perform more rounds
• Perform more reps