A very simple technique where you ask all the students to write down three sentences about themselves. Two of the sentences must be true and one must be a lie (I usually model this first, so they understand). The students may come up with:
I lived in Spain.
I have a car
I don't like vegetables.
The rest of the students then have to guess which sentence is a lie.
This is a great activity as a warm-up or as a filler. Students can also work on their sentence structures when they write the sentences on the board, with their peers helping with error correction.