You might try something like the New Standards portfolio system for students who need some longer term projects. For example, one version (middle school level) has a structure for helping students develop a pure mathematics investigation, conduct a data study, plan and build a physical structure, etc. I've seen something similar that the Boston public schools developed for both grades 5-8 and 9-12. Nice stuff! This type of work would give an additional challenge to selected students without simply pushing them ahead to cover yet another topic one grade ahead in a superficial manner.