We like and use the Michael Clay Thompson language arts materials from Royal Fireworks Press (there are writing, as well as grammar, poetry, and vocabulary books for each level--you needn't do all the books at each level if you're not looking for, say, a poetry course). The first level, the "Island" books, are supposedly for a grade 3 gifted classroom, and adapt well to a homeschool context.
Something else I've tried with Groucho, also from RFWP, is the set of four Aesop workbooks by Kathryn Hegeman ("My books about reading, writing, and thinking"--they're for students anywhere from kindergarten to grade 3.) They're not as unusual, nor as challenging, as the MCT books, but they are nice little books, inexpensive, with a lot of variety of work to do.
A homeschooler I like and trust uses "Write With the Best," which also looks interesting. If I recall correctly, the first one of these says it can be used anywhere from grades 3-12!
Hope that gives you a couple of things to examine, anyway...
peace
minnie
