Exercise is the best way for a lot of people. Whatever you like doing - walking, running, hiking. you don't need to run a marathon in your cell.
We often break up the day with a walk (with toddler on her scooter) after lunchtime. This is something we couldn't do before my husband was working from home. He also goes for a run around 6pm on some days (earlier than if he was working in the office) and we go for a family walk on the other evenings.
Overall, we get to exercise together more than before.
Husband tells me that in the past when he was out of work (and living on his own) in China, the thing that kept him sane was his daily run.