Regardless of how advanced a student's vocabulary may be, there is always room for new words to enhance her writing. With each letter of the alphabet, have a student pick out one unfamiliar word that describes them. Then, for the next few assignments, have her use all 26 new words in context. The idea behind her choosing words about her instead of random ones is to make it more personal, which in turn makes things easier to memorize.
Have the student draw a picture, then have him write about it. Even if he is not artistically inclined, give him a few minutes to come up with something, no matter how simple or vague, since the focus is more on what he will be writing about it. This can work better than presenting him with an already drawn picture or an existing photograph because his familiarity with his own drawing will encourage him to dig deeper when describing it.
Interviewing and profiling someone is a great activity for all writing students. For the novices, it's refreshing to break away from the tired term paper and essay format that is usually all they have known. Advanced students benefit in that the activity is great for teaching and reinforcing several writing skills at once, from summarizing to creative description. Let students interview anyone they wish---it could be a local celebrity or simply a family member---and write a feature profile about them.
Metawriting is the act of writing about writing, and it is especially useful in reinforcing grammar and usage principles. For example, Douglas James Joyce, a teacher consultant with the Denver Writing Project, has his students write well-sourced, one page essays on frequently made mistakes. The method is also useful with many other writing shortcomings as well, from awkward transitions to thin descriptive language.