20 sentences, some of which are grammatically correct and some of which aren't. Put ss into groups and give them the sheet; give them some time to look at the sentences and decide which are correct. Then hold an auction where the ss 'buy' the sentences they think are correct. SS can win some of their money back if they can correct the sentence (obviously you can do the auction any way you want). Basic sentences in pres. perfect, past simple, pres. simple, 1st conditional, some & any etc.