Plyometrics are an excellent substitute if you can't get to weights. These are basically calisthenics with explosive movements. For instance, squats where you explode upward into a jump and try to bring your knees to your chest, or push-ups where you explode upward and clap before catching yourself build strength (and stress the bone, which seems to be beneficial in preventing osteoporosis). Also, adding sprints to the end of your run is beneficial (I really like interval training in general, actually).