Most teachers learn about the grammar auction during their initial training, but just in case you forgot, here it is!
Students are put into pairs and are told that they are in an auction house, where people bid against each other for antiques. The hope is that they buy something which is a genuine antique and worth a lot of money, and that they can pick it up for only a little. What they don't want to do is spend a lot of money on an antique which actually isn't worth much at all. Instead of buying antique furniture, however, the students are bidding for sentences. They have a sheet with twelve sentences on it, six of which are correct (the equivalents of genuine antiques); six of which are wrong (the equivalent of forgeries and completely worthless).
As each sentence comes up, presented by the teacher, the student pairs must guess whether it is correct or incorrect and start bidding for it from the ?5000 that you have generously given them. The bidding goes up and up until someone wins the bid and bang! You strike the table with your imaginary gavel. Then the students are told whether it is correct or not. If it is an incorrect sentence, they can then optionally earn part of their money back by correcting it. The auction continues until all the sentences have been dealt with, or until the students have no more money or until the teacher has no more voice!
This is one way to deal with areas of grammar the student's find particularly difficult, student composition mistakes, or false friends etc.