Here are some links to articles about combined level classrooms:

What works? Research into practice on optimising learning in a combined level classroom
http://www.edu.gov.on.ca/eng/literacynumeracy/inspire/research/Demore.pdf

The following article is more about differentiation in a learning languages classroom, but contains some ideas for superb activities in a combined level class
https://www.actfl.org/sites/default/files/pdfs/TLEsamples/TLE_Aug11_Article.pdf