Be careful if you do decide to set up a 12×12 turnout pen. When my gelding foundered from a shot and had to be on stall rest for a period of time, we were glad to toss him into a small 12x12ish turnout area for the morning stall cleaning. One day I visited to just check the pads on his feet and didn’t turn him out for that period of time, and he apparently got so upset that he was rearing and knocked his water buckets off the wall in frustration. Thankfully he didn’t injure himself, but a horse with a tendon injury might have. Definitely don’t set up a solid turnout routine in the case that one day it won’t be followed.
I agree with an active stall if that is a possibility and she likes to be in on the action. My gelding was unfortunately stuck in the barn all day by himself, with company at night.
I want to say that we tried jolly balls, but they only ever got pushed around in the stall. Horses seem to only like them if they played with them when they were foals/younger. Hanging one off a rafter just led to him being afraid of it, so that was a no-go as well. We couldn’t give him the sweet treats to hang, but I think ones we did try were just ignored.
Hay was really the most important thing, and I would second cutting down grain to a minimum and jacking up the hay.