Topic: grammar / vocabulary.
Materials: student's own mobile phones.
Time: 5 to 15 minutes.
Everyone has a mobile phone. Most students end up clashing with the school protocols against the usage of those in classroom, but you can change that a bit.
Put the students into working pairs. Call out a letter. The students see in their mobiles databank who is the first person with that letter and talk with a partner about that person, for about a minute. When the time is up, change letters. Keep the activity speedy!
It's guaranteed that students would like this and feel at ease about the school protocols against mobile phones in class afterwards.