I like Lukebar's suggestions. Definitely do something where they use their bodies and move around. clapping/tapping stead beat, then doubling it to half notes, then doubling it to whole notes. Have them copy you so they can see how to move. I always make sure to exaggerate so that the whole notes are really big! In other words, your arms are making huge circles because it takes 4 beats to come back to the clap or tap. But then when you do quarters, it's a smaller circle.
Then everyone can sing the song but just show the whole notes with a big circle. You could even have different things for different rhythms:
quarter = snap
half = tap on lap
whole = big circle clap
Old MacDonald would go:
snap snap snap snap snap snap tap
snap snap snap snap BIG
or whatever =)