Using a bench or chair and a medicine ball, try doing 3 sets of 20 OR FOR A GREATER CHALLENGE: do each set until fail, which means not counting. This will challenge your core far more & get your abs burning
Four-Corner Farmer's Walk: Place 2 cones 10 yards apart. Do 1 to 3 sets of the sequence. Rest between sets. (1) Standing at cone, do 8 to 10 bent-over rows. (2) Walk to other cone and do 8 to 10 biceps curls. (3) Walk back to the 1st cone and do 8 to 10 squats. (4) Return to the 2nd cone and do 8 to 10 overhead presses.
Treadmill intervals to burn belly fat - totally doable...highest speed is 7 mph. Great template and you can increase speed/incline/time based on your ability and progress. Running really is great core work, and it burns fat. No one can see your abs if they're covered in fat (duh).