With my running miles curtailed, I've been swimming an average of 6-7 days per week, and getting a great cardiovascular workout.
Recently, I've added rope skipping- what a fabulous, highly underrated activity! If I was a high school or college distance coach, I'd be warming up my runners with 10-15 minutes of skipping before workouts.
Since I am a beginner, I've been doing 10 sets of 100 skips- with 30 seconds recovery. Like a beginning runner tackling a run/walk program, I'll be trying to progress by slowing increasing the length of each skipping interval.
Here is a good summary of reasons why every runner should consider adding rope skipping to their weekly routine (jeffcubos.com)
I'd add that it's also a great aerobic workout and a good way to strengthen muscles and supporting tissues below the knee-including arches, feet, achilles, and calf muscles, perhaps decreasing risk of injury.
This is one additional activity besides swimming, running, and biking that I plan to stick with!