I do (or did before this shut-down) exclusively HIIT on the treadmill for cardio days. I miss running outside but it's too tough on my knees and I have a hard time pacing myself so that's out and I can't swim. Elliptical I never cared for and while I have access to both a regular and stationary bike at home, neither will jack up my pulse rate high enough due to medication keeping it low. Doing interval training not only gave my a good workout, but torched the calories much better than a steady pace run would.
That along with not being able to only do the most basic of muscle maintenance with my limited weight set has really messed up my body these past two months and really brought my mood down to the point there are days I don't even feel like taking a walk outside.