I've been Gifted & Talented co-ordinator at a previous school and my own DD is also on the G&T list for writing, so I have a particular interest in this kind of thing!
First of all the fact that she is already reading and writing for pleasure is perfect in terms of practice. Then as far as possible, with anything you do, focus on it being enjoyable.
Re marking - I'd ask her if she'd like you to. Some children will be happy with this, others not so much! (My DD wouldn't mind, but I wrote stories as a kid myself and would have hated them being marked.) If she says yes, then you could set little targets for improvement.
Or you could suggest she does it herself - mark with a different colour pencil and identify her own strengths and targets. But make sure she is happy with it - if it's "fun" to her then I'd be tempted to leave her to it rather than risk putting her off. If her levels are that high, then what she most needs is to continue practising and loving literacy, since effectively she is already challenging herself.
Other than that, I'd mainly suggest ensuring she reads a wide range of different genres including non fiction and poetry and classics, then discuss with her, esp inferential type qs. She could also have a go at different types of writing/presentation - so she could have a given topic and find out about it (of her choosing). Or write some poems in a certain style. Maybe see if you can get her interested in any particular authors - she could write book reviews or a letter to the author saying why she likes them. You could also do fun word puzzles - there are loads of books available - ones which widen her vocabulary would be particularly good.
The trouble with grammar exercises is you may end up covering stuff she will then do anyway in Y5 and 6, so if you do decide to go that route, check you're not doubling up. She'll probably pick most of it up from her reading anyway, that was how I learned!
Oh you could also try enriching her knowledge in other areas via reading - so for example, parts of History which interest her, or Science?
Hope that helps!