One of my favorite writing exercises is one that I was introduced to about ten years ago. The 15-Sentnece Portrait was developed by the late Wendy Bishop. It is an extremely guided writing exercise. Each of the 15 sentences in the writing assignment has an instruction on what to include in that sentence. It could be a color, a wish, a body part, a specific number of words, etc.
In the abstract, it sounds like a straight jacket, but I have seen it produce some really excellent writing from my students over the years. And this year is no exception. I have an instruction page for it on my class website and even created a step-by-step PowerPoint to guide my students through writing one.
I have shared this idea with others, and have had reports of similar good results. Give it a try and let me know how it works for you.