As a graduate of Purdue, I am rightfully proud of their Online Writing Lab (OWL). Their introduction to paraphrasing is good and they have a set of paraphrase exercises that are very helpful. I usually have the students work in groups.
In the computer lab:
This set of plagiarism exercises lets the students work at their own pace.
This set has a quote and the submitted text in four examples, letting the students talk about these in pairs might be useful.
This is a whole set of links on avoiding plagiarism and academic dishonesty. One of them might be better for you than others.