If the rain is too heavy, you can try running the stairs. Running up and down is a very tiring sport When I was a child, the teacher punished the students and sometimes made us run the stairs. If we are punished more, we will run more. Very physical exercise! It is extreme aerobic exercise, which can increase vital capacity and cardiac power.
You can run on the treadmill in the gym with an umbrella. Although there will be some light rain on the road, it will be good to get to the gym.
Running is not the only aerobic exercise, if the above methods are not perfect. Then you can try another high-intensity aerobic exercise. So you can't find any excuses, you can do it at home, hehe.
I hope it helps you!