Here's an exercise I often use in the first week of term because it is so simple and can produce such amazing results.
The task is to describe a room - either one you know well, perhaps the one you are sitting in right now, or a place straight from the imagination. BUT you have to do it in two different ways.
1) In just one sentence using as many words as you can. See how many you can fit in while still making sense and obeying the usual rules of English. If you scroll down you can see my examples, but if you want inspiration go to Charles Dickens, go to Tom Woolf
2) Same room and this time you are allowed to write as many sentences as you want but each one cannot be longer than SIX words.
Here are my examples
One long sentence
The gilt on the antique picture frames that decorated the walls of the guest bedroom were set on fire by the early morning light that shone through the stained glass window and brought the collection of oil paintings to new, vibrant life while transforming the swathes of muslin draping the four poster bed into an opal waterfall of translucent colour.(60 words)
The light shines through the stained glass. The oil paintings come alive. Dawn sets the picture frames alight. Ancient gilt turns into gold. Muslin turns into water.Try it.
The length of sentences can add texture to your writing, contributing to the atmosphere. They can also add speed.
Try this exercise again but this time describing an action - perhaps someone running away.