Choralnet.com (http://www.choralnet.org/view/222090)
This website has many great resources for handling a choir class. This link specifically takes you to a forum where music teachers post possible lessons to teach when substituting for music. Some are slightly more difficult unless you have a background in music, but many are doable or adaptable for the average teacher!
Fun Music Co.(http://www.funmusiccoimages.com/pdf/substitutelessonideasforthemusicteacher.pdf_)
This is another website that lists off practical, easy lessons for substitute teachers. Some of them require a knowledge of music but quite a few would be manageable even without.
Self-explanatory!