Hand out a list of sentences containing one (or two) incorrections to groups.
Students group and you allot 100 points to each group. They have to bet a number of points (maximum: 10) that they can correct an error.
This is added to, OR SUBTRACTED from, their score if their correction is right, or wrong. (The level of concentration even from students who usually don't worry too much about how correct their English is can be miraculous!)