Not a teacher, but could you go down the route of morals/values, rather than anything specifically religious? Stories/songs about sharing, friendship, looking after one another? Pretty relevant to all religions (and indeed, no religion too).
I've been told asked to sort out Collective Worship assemblies at my school and I don't have a clue where to start. Basically, the Governors want more done for collective worship, more singing, some more 'Thoughts of the Day' etc.
The school is an inner city Comp, with, as you can imagine, a large number of different ethnicities represented there. Can I ask how your school handles this?