This is one of my favorite in-class activities of all time, which I remembered and used recently in a class about writing description/observation. It is the "boring paragraph" assignment. I tend to give it out as a pair activity, and then have students share their rewritten paragraphs in class. It's fun. (It can also be tweaked to be about tone, which is fun too.)

I tell students that they can replace or add ANY words (any part of speech) that they want as long as they maintain the sense of the paragraph. (So, for instance, they can't have the two main characters fly away at the end. That would be changing it too much--)

"The two people left their house and began to make their way down the street. The weather was just as they had expected it would be, and they were dressed appropriately. As they turned the corner to walk along the riverbank, they commented on the small boat that was sailing just ahead of them. It was occupied by a man with a hat on, who was accompanied by a small dog. The traffic rushed by on their left and the boat sailed onwards on their right. As they progressed in the direction of the bridge, they wished they too could be carried along in a car or a boat. Instead they were forced to direct their feet toward their current destination."