Plank
Bodyweight
Get into the same position as a pushup but with your elbows on the ground and under the shoulders. Keep the body straight and tense everything.
       
Side plank
Position body onto one elbow and your feet on top of each other. Keep the body straight.
       
Mountain climbers
Get down on all fours. Alternate the feet quickly back and forth.
       
Jumping lunges
This is like a regular lunge but done more explosively. Switch the feet in mid air.