I'm sorry your poor baby has a broken arm. Right now, it'll probably be a little akward, but not for very long.
My oldest fractured her wrist when she was about the same age, and had to wear a cast for around a month or so. We didn't even know she'd broken it until a week after it happened, despite the fact we'd taken her to the ER and had her X-rayed. The on-call doc read the film wrong, so she went a week with her arm basically broken and unprotected. She was always outside, always playing, had soccer, rode bikes, and always on the go too, but it didn't stop her, even before she had the cast on and her arm was hurting her.
She figured out how to ride her bike, still went outside to play, still did everything pretty normally, she just couldn't write with her right hand, and eating was interesting lol.
Your daughter may even be able to still go to practice as long as she's careful. Should ask the doc, if they didn't mention it, and see if it would be okay.
As far as indoor "quiet" activities, she would probably like a couple new coloring books, maybe even some paint by numbers(my daughter got pretty good at using her left hand for many activities, and can still do it on occasion), and she could make mosaics, and those sun-catcher/stained glass projects.
Also, let her help with cooking. She can do the measuring and stir(you hold the bowl), and if she uses the computer at all, you could change your mouse to left hand clicking. She'd just have to "chicken peck" type with her other hand for any typing.
OH, do you have any of the new Crayola 3-D art supplies? They have the chalk and paint. Get her some of that and tell her to go outside and use her left hand to come up with whatever interesting and different drawings she can, and then put the glasses on.
Anyway, this is all I can think of off the top of my head--hope I've helped. She'll amaze you at just how adaptive she really is with this thing on and what she can still do.
Beware of the stench when it finally comes off. Ewwww, it's nasty! ;-)
Again I'm sorry her arm is broken :o(, and I hope it gets better fast! :o)