Give each child a piece of white construction paper and strips of paper in various rainbow colours (one of each if you have it, or as many colours as you have).
Show them how to glue the coloured strips across the paper in a straight line. If you have older children they can cut out the strips themselves – it's great scissor work.
Once the rainbow pattern is complete, cut out a letter R from the rainbow – and glue onto another piece of construction paper. You've made your own R is for Rainbow!