I am currently teaching Year 5, but I have also done some Year 6 teaching. For your Maths lesson, how about a lesson to do with price reductions in sales? "20% off" or "buy two and get one free"? Or a quarter off the original price? There are also a couple of good Teachers' TV videos about this theme that would give you some more ideas. You could bring in some "real life" objects that are for sale at a discount. This would spark off lots of discussion about how we can know when a shop really is giving us a good deal or when actually we are being ripped off! Fractions and percentages are sometimes a bit difficult for children to understand, so relating them to real situations would make the lesson more meaningful. You could also get the children to discuss different strategies for working out how much money the price reduction would save you. As this lesson will involve money, fractions and percentages, my guess is that it will be tough enough to stretch the most able!