Hi, Alyssa. For a self-care day, try to engage in something that gives you pleasure. Since sitting is painful, take a walk, even if brings some pain.
Even a short walk can lift your spirits and be good for you physically and emotionally. A walk outdoors is better but if not, just walk around the house.
While walking, be in the moment. Think about and maybe silently say what you are doing.
If you can, clean a closet or cupboards in the kitchen. Take your time at it, and tell yourself you're enjoying it.
Also if you can, do some easy exercises. You can watch Youtube videos for easy exercises on the computer or tv which also can keep you active.
I hope these suggestions help.