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Some exercises that require weights can be done without weights. If you don't have time to hit the gym, you can do calisthenics almost anywhere.  To do an air squat, set up as if you were in front of the squat rack at the gym. Put your feet a little more than shoulder width apart, with your toes facing outward. Using the same form you would in weighted squats, lower your hips and push your butt back. To help keep your balance, push your arms out in front of you as you lower down.  Lunges can be performed without weights very easily. The form is exactly the same with or without weights. To do a glute kickback, set up as if you were going to do a push-up, but stay on your knees. Lift one leg off the ground until your thigh is parallel to the ground. Keep your shin perpendicular to the ground and push your leg up. Lower you leg slowly and repeat with the other leg. To do a short bridge, lie flat on your back with the bottoms of your feet on the floor and your knees at about 45 degrees. Keep your shoulders and arms on the floor. Push up using your heels and raise your hips up as high as possible. Hold at the top for two seconds. Slowly lower your body back to the ground.  Once you've mastered the short bridge, try doing a straight bridge. The straight bridge is a lot like a reverse push up. Sitting up with your legs straight in front of you, place your hands near your hips. Push your torso up and squeeze your butt. Hold for a few seconds before lowering back down.  To do a full bridge, lie flat on your back. Bend your knees and place your hands by your head. Lift your entire body off the ground and arch your back. Push your hips into the air and squeeze your legs, butt, and core. Take a deep breath and make sure you give everything a good stretch. Hold for a 1-2 seconds and then lower yourself back down. Lie down on your side and rest your head on your bottom arm. Place your other arm palm down in front of you body. Keep both of your legs straight and lift your top leg about 1 foot off of your bottom leg. Lift your bottom leg to meet your top leg. Slowly lower both legs to the ground. Another variation is to lie flat on your back with your legs straight in front of you. Slowly lift your legs off the ground to a 90-degree angle. Hold for one second and then lower your legs taking care not to let them touch the ground. Running is a great way to tone your legs and butt. Any kind of running is likely to improve your lower body, but sprinting might be the best way.  For an added challenge, run up a hill. Hill sprints will improve your glutes by putting more stress on your hip flexors.  Running is a high impact activity and may not be good for your joints. If you can't run, try using an elliptical machine or a stationary bike. There are numerous exercises that workout your glutes and lower body. Do some research and find some new exercises to incorporate into your routine. Some people find that adding new exercises keeps them motivated to keep working out.  Try out a hip thrust exercise for something a little different. You could also try joining a yoga class. Yoga is an excellent way to strengthen your muscles, increase flexibility, and tone your body.
Add calisthenics to your routine. Try doing bridges. Do leg raises. Go for a run. Experiment with other exercises.