You could try the unique answer game. Break the class up into groups (I have a class of 40 students, so I usually have six groups), give each group some paper. Then ask the class a question (eg. "What is one color in the rainbow?") then each group discusses their answer and writes it on a bit of paper. Make sure everyone understands their answers are a secret from other groups! The teacher collects all of the papers. Then, the teacher reads out the answers. Any group that wrote an answer that no other group had, gets a point.

Questions that only have 5 or six possible answers are good. For example, teachers names, school subjects, characters from the English textbook, and so on. Very simple, and good fun!

Usually can hold the class attention for the whole 50 minute lesson.