You will need: thin purple ribbons, long enough to tie round wrists.
Talk about purple being the colour that a lot of people use to decorate churches with in Advent. Talk about what hope means (this could be linked to the first candle on the Advent wreath!). Discuss the idea that Advent is a time of hope and waiting for Jesus to come again- waiting for a better time when there will be no more death or sickness or crying. Give everyone a purple ribbon and ask them to tie it round the wrist of someone else in the group, praying as they do so that that person will have hope in Jesus in the coming weeks.
This worked really well for a group that covered ages from 0 to adult, though children needed a little help with tying!