This is a very simple idea, but one we've used a few different ways and always got a lot of positive feedback about.

The basic premise is singing out your prayer, using a few lines from an existing Christmas Carol (or other hymn or song would work).  At a number of events (not just at Christmas) we've used the following;

'O come, O come, Emmanuel... (repeat)
Rejoice, rejoice, Emmanuel shall come to you, O broken, hurting world'

and then last weekend we used the following excerpt from God Rest Ye Merry;

'We sing tidings of comfort and joy, comfort and joy,
we sing tidings of comfort and joy.'

Lead sheets for both of these are attached.  After singing one of these once, we've encouraged the congregation to think about situations in the world - you can specify local, personal, or countries, situations or crises in the world.  Get them to picture the person/issue, and then sing their prayer over that situation.  So with the first carol the idea is be praying God would be 'God with them', Emmanuel, in that situation, whilst in the second carol you are praying blessings of God's 'comfort' and joy.  You can give two or three opportunities to think about different situations and then sing into each one. 

As usual, this is an idea you can adapt to suit your situation.  Let us know if you use it in any interesting ways!