Burpees
Stand with your feet hip-width apart and your arms at your sides (a). Push your hips back, bend your knees, and lower your body as deep as you can into a squat, placing your hands on the floor in front of you (b). Kick your legs backward, so that you're now in a pushup position (c). Quickly reverse the movement to return to the starting position. That's one rep. Do 50.
Make it harder: Instead of kicking your legs straight back, work your obliques harder by kicking your legs out to the right, then the left. Alternate sides with each rep.
Prisoner Squats
Stand with your feet hip-width apart and place your fingers on the back of your head, pulling your elbows and shoulders back and sticking your chest out (a). Lower your body as far as you can by pushing your hips back and bending your knees, keeping your torso upright and your core tight (b). Pause, then slowly push yourself back to the starting position. That's one rep. Do 50.
Mountain Climber
Start in a pushup position with your arms completely straight and your body forming a straight line from your head to your ankles (a). Lift your right foot off the floor and slowly raise your knee as close to your right elbow as possible, without changing your lower-back posture (b). Return to the starting position. That's one rep. Repeat with the other leg, and continue alternating until you've completed 25 reps on each side.