All good ideas, I found that strenuous yoga was really nice too. Get into that mindset and do it. The meditative aspect can be quite enjoyable once you make it routine. Might help you with the lack of fresh air. Also, HIIT, as many others have said. I like the mindfulness, core strength, and flexibility gained from the former and the cardio & muscle maintenance or build up from the ladder. They can both be done in the same day too if you want to wake up and do the first, take a bit to cool down, and knock the HIIT out before bed... I've done that and it kicked my ass. Feels good to be flexible and strong.