Rainy day cardio exercises?

Asked my mom how she felt about me investing in a treadmill for days like today when it's non-stop pouring rain out and I can't go running... and she said we'd "talk about it later".

So in the meantime... does anyone have any good stand-in exercises? I didn't run yesterday because I couldn't without doing damage to myself, and now that I'm in functioning condition again... the weather isn't cooperating. It's annoying.
Rainy day cardio exercises?
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