This is a very interesting activity that most students enjoy.
Divide the class in groups of three to five. The idea is to make them create a radio show. Each student should talk about a subject.
For instance, they can read the news of the day (from a newspaper or invented by themselves), talk about a sports match, give the weather forecast, report on a movie or a book, read a poetry or make an interview to another student (pretending to be someone important).
The show must not last more than five to ten minutes.
You can record on tape the whole activity and make the students hear themselves to correct their own mistakes.