The fun thing I often take into classes is the bopit which is an excellent toy to create silence and involvement - teams playing against one another.
I will include a couple of lesson plans eventually. I never want to be seen as part of the staff and 'just another teacher' so try to restrict myself to no more than 3 lessons for a class per term. This helps keep it 'fresh'.
With older students who are learning for GCSE encouraging discussion is important. The old formula of never asking questions that can be answered with 'yes' or 'no' applies.
Again it is my firm conviction that quantity of educational content is not the be all and end all of a class. If you and the students are having fun in the learning situation more will be retained and enthusiasm maintained and the kids will want to come back for more.