Give each of your students a wipe-clean flash card with the word "wrong" written on it. Write a simple melody on your chalkboard or on separate pieces of paper. Play the melody on your instrument but make a deliberate mistake. Have the children raise their flash cards when they hear the mistake. Start off by making obvious mistakes, such as including notes that are clearly out of tune or deviating from a well-known melody. Increase the challenge by making the mistakes more subtle, such as not leaving a rest where there should be one. This game encourages your students to listen critically.
Write out one rhythm for each child in your class. Include a key that denotes the value of each rhythmical symbol, such as quarter notes, eighth notes and rests. Have each child clap out their rhythm in turn. Award a prize for the most accurate recital. Invert the game by having each student clap out a rhythm while the rest of the class writes down the notes. Award a prize for the most accurate notation.
Notate a melody and play it, but miss one note. Have the children raise their hands when they identify that you've missed a note from the melody. Invite the first student who raised his hand to identify the missing note, either by calling out the note name or playing the complete melody on his instrument.
Split your class into two teams. Write down a list of simple chords, such as C major, A minor, D major and G major. Hand out sheets of paper, each with a note from one of the chords. There should be a sheet for every note in every chord, plus a few "red herrings" that don't appear in any of the chords. Have each team work together to assemble the notes into groups of three, to make up the notes in the chords that you've listed. For example, C, E and G make up C major.
This is a little more advanced. Give half of your class a sheet with "key note" on it and the other half a sheet with the "harmony" note. Organize the sheets so that each key note has a corresponding harmony. For example, if using the major third harmony, C needs an E, G needs a B and D needs an F sharp. Split the group into opposing rows, tell them the harmony they are looking for and have them find their "harmony partner." Hand out copies of the major scales if the children are not yet familiar with harmonies.