You may want to strengthen your core to help support your back. Or you may want to strengthen your glutes to help you run better. Or you may want to work on your upper body strength so you can carry your kids or grandkids more easily. Choose a new strength-training exercise that will target your chosen muscle group. A certified personal trainer can show you the proper form to use. Doing the exercise the right way can help you stay safe and avoid injury.
Make sure to talk with your doctor before starting any new exercise routine.