In Brief: 
Students play the favorite old American card game, War, except rhythm playing cards are used instead of the familiar king, queen, ace, etc. In short two students each turn over one card from their pile. Whomever has the largest value car wins both cards. When cards are the same value, there's a war and the stakes are higher.
Articulate the relative values of notes and rests
Prerequisite Concepts: 
Fluency with the relative values of whole, half, quarter, eighth and sixteenth notes, dotted notes and rests
Teacher's Role: 
Demonstrate game. While students play, you assess and assist when needed.
Play for as much time as you have. Good game for a quick review.